feat(core-api): implement abstract data transformer architecture

- **Core API**: Introduced `IDataTransformer` interface and `BaseDataTransformer` abstract class. - **Data Services**: Updated `BaseRemoteDataServices` and `DataServicesConfig` to support optional transformer injection. CRUD methods now auto-transform data mapping (DTO <-> Entity) when a transformer is provided. - **Generics**: Updated `CommonRemoteDataServices` to forward the `TDTO` generic for strict type safety. Fixed `ApiResponse<T>` casting and excess property type checks. - **Showcase/Samples**: Created `BookingTransformer` (snake_case to camelCase mapping) and injected it into `booking.data-services.ts`. - **Advanced Showcase**: Added `AdvancedBookingTransformer` and `AdvancedBookingService` to demonstrate extending base transformers with custom methods (e.g., `getAvailabilityChart`). - **Documentation**: Added comprehensive developer guide at `apps/docs-dev/src/packages/core-api/transformers.md`. - **Testing**: Added 16 new unit tests for transformer integrations (identity and mock scenarios). Note: The implementation is 100% backward compatible. All 49/49 tests pass and the TypeScript typecheck is entirely clean.
2026-07-01 11:13:09 +07:00
parent 7130eb3fe3
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@@ -0,0 +1,323 @@
+# Data Transformers
+
+> **Purpose:** Separate data transformation logic from API call logic, enabling clean DTO ↔ Entity mapping with type safety.
+>
+> **Location:** `packages/core-api/src/data-services/base-data.transformer.ts`
+
+---
+
+## Why Data Transformers?
+
+In enterprise applications, the shape of data returned by the API (DTOs) often differs from the shape used in the frontend (Domain Entities). Common differences include:
+
+| API (DTO)                      | Frontend (Entity)            |
+| ------------------------------ | ---------------------------- |
+| `snake_case` field names       | `camelCase` field names      |
+| Deeply nested structures       | Flattened/normalized shapes  |
+| Raw ISO date strings           | Parsed `Date` objects        |
+| No computed fields             | Derived/computed properties  |
+| Backend-specific enums         | Frontend-friendly enums      |
+
+Without transformers, this mapping logic leaks into components, hooks, and services — violating the **Single Responsibility Principle** and making the codebase harder to test and maintain.
+
+### Benefits
+
+- **Separation of Concerns** — Transformation logic lives in one place, not scattered across components
+- **Testability** — Transformers are pure functions, trivially unit-testable
+- **Reusability** — Same transformer can be used across multiple services or contexts
+- **Type Safety** — Two generic parameters (`TEntity`, `TDTO`) enforce correct mapping at compile time
+- **Backward Compatible** — Transformers are optional; existing services work unchanged
+
+---
+
+## Architecture
+
+```mermaid
+graph LR
+    classDef api fill:#f59e0b,stroke:#b45309,color:#fff
+    classDef transformer fill:#6366f1,stroke:#4338ca,color:#fff
+    classDef entity fill:#10b981,stroke:#047857,color:#fff
+    classDef service fill:#3b82f6,stroke:#2563eb,color:#fff
+
+    API["🌐 REST API<br/>(snake_case DTOs)"]:::api
+    SVC["BaseRemoteDataServices<br/>(execute, getOne, getMany, ...)"]:::service
+    TFM["Data Transformer<br/>(transformToEntity / transformToDTO)"]:::transformer
+    ENT["Domain Entity<br/>(camelCase, computed fields)"]:::entity
+
+    API -->|"Response (DTO)"| SVC
+    SVC -->|"dto"| TFM
+    TFM -->|"entity"| ENT
+
+    ENT -->|"entity"| TFM
+    TFM -->|"dto"| SVC
+    SVC -->|"Request (DTO)"| API
+```
+
+**Data flows:**
+- **API → Frontend:** Response DTO → `transformToEntity()` → Domain Entity
+- **Frontend → API:** Domain Entity → `transformToDTO()` → Request DTO
+
+---
+
+## Quick Start
+
+### 1. Define Your Types
+
+```typescript
+// Domain Entity (what your UI uses)
+interface BookingEntity extends BaseEntity {
+  bookingCode: string;
+  customerName: string;
+  checkInDate: string;
+}
+
+// API DTO (what the backend returns)
+interface BookingDTO {
+  id?: string;
+  booking_code: string;
+  customer_name: string;
+  check_in_date: string;
+}
+```
+
+### 2. Create a Transformer
+
+```typescript
+import { BaseDataTransformer } from '@repo/core-api/data-services';
+
+class BookingTransformer extends BaseDataTransformer<BookingEntity, BookingDTO> {
+  transformToEntity(dto: BookingDTO): BookingEntity {
+    return {
+      id: dto.id,
+      bookingCode: dto.booking_code,
+      customerName: dto.customer_name,
+      checkInDate: dto.check_in_date,
+    };
+  }
+
+  transformToDTO(entity: BookingEntity): BookingDTO {
+    return {
+      id: entity.id,
+      booking_code: entity.bookingCode,
+      customer_name: entity.customerName,
+      check_in_date: entity.checkInDate,
+    };
+  }
+}
+```
+
+### 3. Inject into Data Services
+
+```typescript
+import { CommonRemoteDataServices } from '@repo/core-api/data-services';
+
+const bookingServices = new CommonRemoteDataServices<BookingEntity, BookingDTO>(
+  apiClient,
+  {
+    apiUrl: '/bookings',
+    moduleKey: 'BOOKING',
+    transformer: new BookingTransformer(),
+  },
+);
+
+// Now all CRUD methods automatically transform:
+const { data } = await bookingServices.getOne('42');
+// data is BookingEntity (camelCase) ✓
+
+await bookingServices.create({ bookingCode: 'BK001', customerName: 'Alice', ... });
+// Payload is sent as { booking_code: 'BK001', customer_name: 'Alice', ... } ✓
+```
+
+---
+
+## Interface Reference
+
+### `IDataTransformer<TEntity, TDTO>`
+
+The minimal contract for bidirectional data transformation.
+
+```typescript
+interface IDataTransformer<TEntity, TDTO> {
+  transformToEntity(dto: TDTO): TEntity;
+  transformToDTO(entity: TEntity): TDTO;
+
+  // Optional operation-specific hooks
+  transformGetOneResponse?(dto: TDTO): TEntity;
+  transformGetManyResponse?(dtos: TDTO[]): TEntity[];
+  transformCreatePayload?(entity: Partial<TEntity>): Partial<TDTO>;
+  transformEditPayload?(entity: Partial<TEntity>): Partial<TDTO>;
+}
+```
+
+### `BaseDataTransformer<TEntity, TDTO>`
+
+Abstract class implementing `IDataTransformer` with sensible defaults.
+
+| Method                    | Default Behavior                         | Override When                              |
+| ------------------------- | ---------------------------------------- | ------------------------------------------ |
+| `transformToEntity`       | Identity cast (passthrough)              | Always — this is the core mapping          |
+| `transformToDTO`          | Identity cast (passthrough)              | Always — this is the core mapping          |
+| `transformGetOneResponse` | Delegates to `transformToEntity`         | `getOne` needs computed/derived fields     |
+| `transformGetManyResponse`| Maps each item via `transformToEntity`   | List responses need bulk transformations   |
+| `transformCreatePayload`  | Delegates to `transformToDTO`            | Create payloads need special handling (e.g., strip IDs) |
+| `transformEditPayload`    | Delegates to `transformToDTO`            | Edit payloads differ from create           |
+
+---
+
+## Integration with `BaseRemoteDataServices`
+
+When a transformer is injected via `DataServicesConfig.transformer`, the base service methods automatically apply transformations:
+
+| Service Method | Transformer Hook Used            | Direction       |
+| -------------- | -------------------------------- | --------------- |
+| `getOne()`     | `transformGetOneResponse()`      | Response → Entity |
+| `getMany()`    | `transformGetManyResponse()`     | Response → Entity |
+| `create()`     | `transformCreatePayload()`       | Entity → DTO     |
+| `edit()`       | `transformEditPayload()`         | Entity → DTO     |
+| `delete()`     | None (no data transformation)    | —               |
+| `customRequest()` | None (manual transformation)  | —               |
+
+> **Important:** If no transformer is injected, all methods behave exactly as before — data passes through unchanged. This ensures 100% backward compatibility.
+
+---
+
+## Advanced: Extending Transformers
+
+For domain-specific features that go beyond standard CRUD, you can extend both the transformer and the data service.
+
+### Extended Transformer
+
+```typescript
+// advanced-booking.transformer.ts
+import { BookingTransformer } from './booking.transformer';
+
+interface AvailabilityChartRawData {
+  dates: Array<{
+    date_iso: string;
+    available_rooms: number;
+    occupancy_rate: number;
+  }>;
+}
+
+interface AvailabilityChartData {
+  dataPoints: Array<{
+    label: string;
+    availableRooms: number;
+    isHighDemand: boolean;
+  }>;
+}
+
+class AdvancedBookingTransformer extends BookingTransformer {
+  // All standard CRUD mappings are inherited ✓
+
+  // Add custom transformation for non-CRUD data
+  transformAvailabilityChart(rawData: AvailabilityChartRawData): AvailabilityChartData {
+    return {
+      dataPoints: rawData.dates.map((item) => ({
+        label: new Date(item.date_iso).toLocaleDateString('en-US', {
+          weekday: 'short',
+          month: 'short',
+          day: 'numeric',
+        }),
+        availableRooms: item.available_rooms,
+        isHighDemand: item.occupancy_rate > 80,
+      })),
+    };
+  }
+}
+```
+
+### Extended Data Service
+
+```typescript
+// advanced-booking.data-services.ts
+import { BaseRemoteDataServices } from '@repo/core-api/data-services';
+
+class AdvancedBookingDataServices extends BaseRemoteDataServices<BookingEntity, BookingDTO> {
+  private readonly advancedTransformer: AdvancedBookingTransformer;
+
+  constructor() {
+    const transformer = new AdvancedBookingTransformer();
+    super(apiClient, {
+      apiUrl: '/bookings',
+      moduleKey: 'BOOKING',
+      transformer,
+    });
+    this.advancedTransformer = transformer;
+  }
+
+  // Custom method using the extended transformer
+  async getAvailabilityChart(params: {
+    startDate: string;
+    endDate: string;
+  }): Promise<ApiResponse<AvailabilityChartData>> {
+    const response = await this.customRequest<AvailabilityChartRawData>({
+      url: '/bookings/availability-chart',
+      method: 'GET',
+      params: { start_date: params.startDate, end_date: params.endDate },
+    });
+
+    return {
+      data: this.advancedTransformer.transformAvailabilityChart(response.data),
+      status: response.status,
+    };
+  }
+}
+
+export const advancedBookingServices = new AdvancedBookingDataServices();
+```
+
+---
+
+## Migration Guide
+
+Adding transformers to existing services requires **zero breaking changes**:
+
+### Step 1: Create a Transformer
+
+```typescript
+class MyTransformer extends BaseDataTransformer<MyEntity, MyDTO> {
+  transformToEntity(dto: MyDTO): MyEntity { /* ... */ }
+  transformToDTO(entity: MyEntity): MyDTO { /* ... */ }
+}
+```
+
+### Step 2: Add a Second Generic Parameter
+
+```diff
+- const services = new CommonRemoteDataServices<MyEntity>(apiClient, {
+ const services = new CommonRemoteDataServices<MyEntity, MyDTO>(apiClient, {
+    apiUrl: '/my-endpoint',
+   transformer: new MyTransformer(),
+  });
+```
+
+### Step 3: (Optional) Override Operation-Specific Hooks
+
+```typescript
+class MyTransformer extends BaseDataTransformer<MyEntity, MyDTO> {
+  transformToEntity(dto: MyDTO): MyEntity { /* ... */ }
+  transformToDTO(entity: MyEntity): MyDTO { /* ... */ }
+
+  // Only override if getOne needs special handling
+  override transformGetOneResponse(dto: MyDTO): MyEntity {
+    const entity = this.transformToEntity(dto);
+    return { ...entity, computedField: derive(dto) };
+  }
+}
+```
+
+> **Existing services without transformers are completely unaffected.** The `TDTO` generic defaults to `TEntity`, and the `transformer` config property defaults to `undefined`.
+
+---
+
+## Sample Implementation
+
+A full working example is available in the showcase booking feature:
+
+| File | Description |
+| ---- | ----------- |
+| `apps/web/.../booking/data/booking.transformer.ts` | Basic transformer with snake_case ↔ camelCase mapping |
+| `apps/web/.../booking/data/booking.data-services.ts` | Data service with injected transformer |
+| `apps/web/.../booking/data/advanced-booking.transformer.ts` | Extended transformer with custom chart method |
+| `apps/web/.../booking/data/advanced-booking.data-services.ts` | Extended service with custom `getAvailabilityChart()` |
@@ -0,0 +1,118 @@
+import { BaseRemoteDataServices } from '@repo/core-api/data-services';
+import type { ApiResponse } from '@repo/core-api/http-client';
+import { apiClient } from '../../../../../../core/lib/api-client';
+import type { BookingEntity } from './booking.data-services';
+import type { BookingDTO } from './booking.transformer';
+import {
+  AdvancedBookingTransformer,
+  type AvailabilityChartRawData,
+  type AvailabilityChartData,
+} from './advanced-booking.transformer';
+
+// ─── Advanced Booking Data Services ─────────────────────────────
+
+/**
+ * Extended booking data services with custom methods for
+ * advanced booking features beyond standard CRUD.
+ *
+ * Extends {@link BaseRemoteDataServices} directly (instead of using
+ * `CommonRemoteDataServices`) to add domain-specific methods like
+ * `getAvailabilityChart()`.
+ *
+ * Uses {@link AdvancedBookingTransformer} which provides:
+ * - All standard DTO ↔ Entity mappings (inherited from BookingTransformer)
+ * - Custom `transformAvailabilityChart()` for chart data
+ * - Enhanced `transformGetManyResponse()` with status normalization
+ *
+ * @example
+ * ```ts
+ * // Standard CRUD (inherited, with transformer)
+ * const { data: bookings } = await advancedBookingServices.getMany();
+ * const { data: booking } = await advancedBookingServices.getOne('42');
+ *
+ * // Custom method for chart data
+ * const { data: chartData } = await advancedBookingServices.getAvailabilityChart({
+ *   startDate: '2026-07-01',
+ *   endDate: '2026-07-31',
+ * });
+ * ```
+ */
+class AdvancedBookingDataServices extends BaseRemoteDataServices<BookingEntity, BookingDTO> {
+  /**
+   * The concrete advanced transformer instance.
+   *
+   * Stored separately from the base `transformer` property
+   * to access custom methods (like `transformAvailabilityChart`)
+   * that aren't part of the `IDataTransformer` interface.
+   */
+  private readonly advancedTransformer: AdvancedBookingTransformer;
+
+  constructor() {
+    const advancedTransformer = new AdvancedBookingTransformer();
+
+    super(apiClient, {
+      apiUrl: '/bookings',
+      moduleKey: 'BOOKING',
+      transformer: advancedTransformer,
+    });
+
+    this.advancedTransformer = advancedTransformer;
+  }
+
+  // ─── Custom Methods ─────────────────────────────────────────
+
+  /**
+   * Fetch the availability chart data for a given date range.
+   *
+   * Calls the `/bookings/availability-chart` endpoint and transforms
+   * the raw API response into a UI-friendly chart format using
+   * {@link AdvancedBookingTransformer.transformAvailabilityChart}.
+   *
+   * @param params - Date range parameters for the chart query
+   * @param params.startDate - Start date (ISO format, e.g., '2026-07-01')
+   * @param params.endDate   - End date (ISO format, e.g., '2026-07-31')
+   * @returns Transformed chart data ready for UI rendering
+   *
+   * @example
+   * ```ts
+   * const { data } = await advancedBookingServices.getAvailabilityChart({
+   *   startDate: '2026-07-01',
+   *   endDate: '2026-07-31',
+   * });
+   *
+   * // data.dataPoints → Array of chart-ready data points
+   * // data.summary    → Aggregated metrics for the period
+   * ```
+   */
+  async getAvailabilityChart(params: {
+    startDate: string;
+    endDate: string;
+  }): Promise<ApiResponse<AvailabilityChartData>> {
+    const response = await this.customRequest<AvailabilityChartRawData>({
+      url: '/bookings/availability-chart',
+      method: 'GET',
+      params: {
+        start_date: params.startDate,
+        end_date: params.endDate,
+      },
+    });
+
+    return {
+      data: this.advancedTransformer.transformAvailabilityChart(response.data),
+      status: response.status,
+    };
+  }
+}
+
+// ─── Singleton Export ────────────────────────────────────────────
+
+/**
+ * Pre-configured advanced booking data services instance.
+ *
+ * Use this when you need both standard CRUD operations and
+ * custom methods like `getAvailabilityChart()`.
+ *
+ * For standard CRUD-only usage, prefer `bookingServices` from
+ * `booking.data-services.ts` instead.
+ */
+export const advancedBookingServices = new AdvancedBookingDataServices();
@@ -0,0 +1,159 @@
+import { BookingTransformer } from './booking.transformer';
+import type { BookingDTO } from './booking.transformer';
+import type { BookingEntity } from './booking.data-services';
+
+// ─── Advanced Types ─────────────────────────────────────────────
+
+/**
+ * Raw availability chart data as returned by the API.
+ *
+ * The backend returns a flat structure with snake_case keys
+ * and ISO date strings. This needs to be transformed into
+ * a more UI-friendly shape for chart rendering.
+ */
+export interface AvailabilityChartRawData {
+  dates: Array<{
+    date_iso: string;
+    available_rooms: number;
+    total_rooms: number;
+    occupancy_rate: number;
+    revenue_per_room: number;
+  }>;
+  summary: {
+    avg_occupancy_rate: number;
+    total_revenue: number;
+    period_start: string;
+    period_end: string;
+  };
+}
+
+/**
+ * UI-friendly availability chart data.
+ *
+ * Pre-computed for direct rendering in chart components
+ * with camelCase fields, formatted labels, and derived metrics.
+ */
+export interface AvailabilityChartData {
+  /** Data points ready for chart rendering. */
+  dataPoints: Array<{
+    /** Formatted date label (e.g., 'Mon, Jul 1'). */
+    label: string;
+    /** ISO date string for programmatic use. */
+    dateISO: string;
+    /** Number of rooms available. */
+    availableRooms: number;
+    /** Total room capacity. */
+    totalRooms: number;
+    /** Occupancy rate as a percentage (0-100). */
+    occupancyRate: number;
+    /** Revenue per available room. */
+    revenuePerRoom: number;
+    /** Whether the day is a high-demand day (>80% occupancy). */
+    isHighDemand: boolean;
+  }>;
+  /** Aggregated summary metrics for the period. */
+  summary: {
+    averageOccupancy: number;
+    totalRevenue: number;
+    periodStart: string;
+    periodEnd: string;
+    /** Number of high-demand days in the period. */
+    highDemandDays: number;
+  };
+}
+
+// ─── Advanced Booking Transformer ───────────────────────────────
+
+/**
+ * Extended booking transformer with additional custom methods
+ * for non-CRUD data transformations.
+ *
+ * Inherits all standard DTO ↔ Entity mapping from
+ * {@link BookingTransformer} and adds domain-specific
+ * transformations for advanced features like availability charts.
+ *
+ * **When to extend vs. create new:**
+ * - Extend when the new transformer shares the same entity/DTO pair
+ *   and you need additional transformation methods
+ * - Create a new transformer when the entity/DTO types are different
+ *
+ * @example
+ * ```ts
+ * const transformer = new AdvancedBookingTransformer();
+ *
+ * // Standard CRUD mapping (inherited)
+ * const entity = transformer.transformToEntity(bookingDTO);
+ *
+ * // Custom chart transformation (new)
+ * const chartData = transformer.transformAvailabilityChart(rawChartData);
+ * ```
+ */
+export class AdvancedBookingTransformer extends BookingTransformer {
+  /**
+   * Transform raw availability chart data from the API into a
+   * UI-friendly format for chart rendering.
+   *
+   * Performs the following transformations:
+   * 1. Maps snake_case fields to camelCase
+   * 2. Formats date strings into human-readable labels
+   * 3. Computes derived `isHighDemand` flag (>80% occupancy)
+   * 4. Aggregates `highDemandDays` count in the summary
+   *
+   * @param rawData - Raw chart data from the `/bookings/availability-chart` endpoint
+   * @returns Transformed chart data ready for UI rendering
+   */
+  transformAvailabilityChart(rawData: AvailabilityChartRawData): AvailabilityChartData {
+    const HIGH_DEMAND_THRESHOLD = 80;
+
+    const dataPoints = rawData.dates.map((item) => {
+      const date = new Date(item.date_iso);
+      const isHighDemand = item.occupancy_rate > HIGH_DEMAND_THRESHOLD;
+
+      return {
+        label: date.toLocaleDateString('en-US', {
+          weekday: 'short',
+          month: 'short',
+          day: 'numeric',
+        }),
+        dateISO: item.date_iso,
+        availableRooms: item.available_rooms,
+        totalRooms: item.total_rooms,
+        occupancyRate: item.occupancy_rate,
+        revenuePerRoom: item.revenue_per_room,
+        isHighDemand,
+      };
+    });
+
+    const highDemandDays = dataPoints.filter((dp) => dp.isHighDemand).length;
+
+    return {
+      dataPoints,
+      summary: {
+        averageOccupancy: rawData.summary.avg_occupancy_rate,
+        totalRevenue: rawData.summary.total_revenue,
+        periodStart: rawData.summary.period_start,
+        periodEnd: rawData.summary.period_end,
+        highDemandDays,
+      },
+    };
+  }
+
+  /**
+   * Enhanced getMany response that also normalizes status values.
+   *
+   * Demonstrates overriding an inherited hook to add
+   * additional processing on top of the base transformation.
+   *
+   * @param dtos - Array of booking DTOs from the API
+   * @returns Transformed entities with normalized status
+   */
+  override transformGetManyResponse(dtos: BookingDTO[]): BookingEntity[] {
+    return super.transformGetManyResponse(dtos).map((entity) => ({
+      ...entity,
+      // Normalize 'cancelled' vs 'canceled' from different API versions
+      status: entity.status === ('canceled' as BookingEntity['status'])
+        ? 'cancelled'
+        : entity.status,
+    }));
+  }
+}
@@ -1,6 +1,8 @@
 import { CommonRemoteDataServices } from '@repo/core-api/data-services';
 import type { BaseEntity } from '@repo/core-api/data-services';
 import { apiClient } from '../../../../../../core/lib/api-client';
+import { BookingTransformer } from './booking.transformer';
+import type { BookingDTO } from './booking.transformer';

 // ─── Domain Entity ──────────────────────────────────────────────

@@ -22,19 +24,35 @@ export interface BookingEntity extends BaseEntity {
 // ─── Data Services Instance ─────────────────────────────────────

 /**
- * Booking data services — wired to the enterprise `apiClient`.
+ * Booking data services — wired to the enterprise `apiClient`
+ * with automatic DTO ↔ Entity transformation.
 *
 * All requests flow through the full interceptor chain:
 * Faro tracing → Bearer token injection → ApiError normalization.
 *
+ * The injected {@link BookingTransformer} automatically:
+ * - Maps snake_case API responses to camelCase entities on `getOne`/`getMany`
+ * - Maps camelCase entity payloads to snake_case DTOs on `create`/`edit`
+ * - Strips `id` from create payloads
+ * - Computes `durationNights` on `getOne` responses
+ *
 * @example
 * ```ts
 * const { data } = await bookingServices.getMany({ params: { page: 1 } });
+ * // data is BookingEntity[] with camelCase fields
+ *
 * const { data: booking } = await bookingServices.getOne('42');
+ * // booking is BookingEntity with computed durationNights
+ *
+ * await bookingServices.create({ bookingCode: 'BK001', customerName: 'Alice', ... });
+ * // Payload is automatically transformed to { booking_code: 'BK001', customer_name: 'Alice', ... }
+ *
 * await bookingServices.confirmProcessTransaction('42');
 * ```
 */
-export const bookingServices = new CommonRemoteDataServices<BookingEntity>(apiClient, {
+export const bookingServices = new CommonRemoteDataServices<BookingEntity, BookingDTO>(apiClient, {
  apiUrl: '/bookings',
  moduleKey: 'BOOKING',
+  transformer: new BookingTransformer(),
 });
+
@@ -0,0 +1,123 @@
+import { BaseDataTransformer } from '@repo/core-api/data-services';
+import type { BookingEntity } from './booking.data-services';
+
+// ─── Booking DTO (API Response Shape) ───────────────────────────
+
+/**
+ * Raw booking data as returned by the API.
+ *
+ * Uses snake_case field names matching the backend's JSON serialization.
+ * This DTO is never used directly in UI components — it is transformed
+ * into a {@link BookingEntity} by the {@link BookingTransformer}.
+ */
+export interface BookingDTO {
+  id?: string;
+  booking_code: string;
+  customer_name: string;
+  check_in_date: string;
+  check_out_date: string;
+  status: 'pending' | 'confirmed' | 'cancelled';
+  total_amount: number;
+}
+
+// ─── Booking Transformer ────────────────────────────────────────
+
+/**
+ * Transforms between the API's `BookingDTO` (snake_case) and
+ * the frontend's `BookingEntity` (camelCase).
+ *
+ * Handles:
+ * - Field name mapping (snake_case ↔ camelCase)
+ * - Computed field derivation (e.g., `durationNights` on `getOne`)
+ * - Payload sanitization (e.g., stripping `id` on create)
+ *
+ * @example
+ * ```ts
+ * const transformer = new BookingTransformer();
+ *
+ * // API response → Domain entity
+ * const entity = transformer.transformToEntity({
+ *   id: '42',
+ *   booking_code: 'BK042',
+ *   customer_name: 'Alice',
+ *   check_in_date: '2026-07-01',
+ *   check_out_date: '2026-07-03',
+ *   status: 'confirmed',
+ *   total_amount: 500000,
+ * });
+ * // → { id: '42', bookingCode: 'BK042', customerName: 'Alice', ... }
+ * ```
+ */
+export class BookingTransformer extends BaseDataTransformer<BookingEntity, BookingDTO> {
+  /**
+   * Map an API booking DTO to a frontend booking entity.
+   *
+   * @param dto - Raw booking data from the API
+   * @returns Mapped booking entity with camelCase fields
+   */
+  override transformToEntity(dto: BookingDTO): BookingEntity {
+    return {
+      id: dto.id,
+      bookingCode: dto.booking_code,
+      customerName: dto.customer_name,
+      checkInDate: dto.check_in_date,
+      checkOutDate: dto.check_out_date,
+      status: dto.status,
+      totalAmount: dto.total_amount,
+    };
+  }
+
+  /**
+   * Map a frontend booking entity to an API booking DTO.
+   *
+   * @param entity - Booking entity from the frontend
+   * @returns Mapped booking DTO with snake_case fields
+   */
+  override transformToDTO(entity: BookingEntity): BookingDTO {
+    return {
+      id: entity.id,
+      booking_code: entity.bookingCode,
+      customer_name: entity.customerName,
+      check_in_date: entity.checkInDate,
+      check_out_date: entity.checkOutDate,
+      status: entity.status,
+      total_amount: entity.totalAmount,
+    };
+  }
+
+  /**
+   * Transform a single booking response with computed fields.
+   *
+   * Adds `durationNights` as a derived convenience field
+   * that is only relevant when viewing a single booking detail.
+   *
+   * @param dto - Raw booking DTO from the API
+   * @returns Booking entity with computed fields
+   */
+  override transformGetOneResponse(dto: BookingDTO): BookingEntity {
+    const entity = this.transformToEntity(dto);
+    const checkIn = new Date(dto.check_in_date);
+    const checkOut = new Date(dto.check_out_date);
+    const durationMs = checkOut.getTime() - checkIn.getTime();
+    const durationNights = Math.max(0, Math.ceil(durationMs / (1000 * 60 * 60 * 24)));
+
+    return {
+      ...entity,
+      // Attach computed field via type assertion since
+      // durationNights is a view-layer convenience
+      ...(durationNights > 0 ? { durationNights } : {}),
+    };
+  }
+
+  /**
+   * Strip `id` from create payloads since the backend generates IDs.
+   *
+   * @param entity - Partial booking entity from the create form
+   * @returns Sanitized DTO payload without `id`
+   */
+  override transformCreatePayload(entity: Partial<BookingEntity>): Partial<BookingDTO> {
+    const dto = this.transformToDTO(entity as BookingEntity);
+    const { id: _, ...rest } = dto;
+    return rest;
+  }
+}
@@ -0,0 +1,221 @@
+import type { BaseEntity } from './types';
+
+// ─── Core Transformer Interface ─────────────────────────────────
+
+/**
+ * Minimal contract for bidirectional data transformation
+ * between API Data Transfer Objects (DTOs) and domain entities.
+ *
+ * Implement this interface when you only need the two core
+ * mapping methods without operation-specific hooks.
+ *
+ * @typeParam TEntity - The frontend domain entity type
+ * @typeParam TDTO    - The API response/request DTO shape
+ *
+ * @example
+ * ```ts
+ * class UserTransformer implements IDataTransformer<UserEntity, UserDTO> {
+ *   transformToEntity(dto: UserDTO): UserEntity {
+ *     return { id: dto.id, fullName: `${dto.first_name} ${dto.last_name}` };
+ *   }
+ *   transformToDTO(entity: UserEntity): UserDTO {
+ *     const [first, ...rest] = entity.fullName.split(' ');
+ *     return { id: entity.id, first_name: first, last_name: rest.join(' ') };
+ *   }
+ * }
+ * ```
+ */
+export interface IDataTransformer<
+  TEntity extends BaseEntity = BaseEntity,
+  TDTO = TEntity,
+> {
+  /**
+   * Transform an API DTO into a domain entity.
+   *
+   * Called after receiving data from the API. Use this to map
+   * snake_case fields to camelCase, flatten nested structures,
+   * compute derived fields, or apply any normalization.
+   *
+   * @param dto - Raw data from the API response
+   * @returns The mapped domain entity
+   */
+  transformToEntity(dto: TDTO): TEntity;
+
+  /**
+   * Transform a domain entity into an API DTO.
+   *
+   * Called before sending data to the API. Use this to map
+   * camelCase fields to snake_case, restructure nested objects,
+   * or strip frontend-only computed fields.
+   *
+   * @param entity - Domain entity from the frontend
+   * @returns The mapped DTO for the API request
+   */
+  transformToDTO(entity: TEntity): TDTO;
+
+  // ─── Optional Operation-Specific Hooks ──────────────────────
+
+  /**
+   * Transform the response of a `getOne()` call.
+   * If not provided, falls back to `transformToEntity`.
+   */
+  transformGetOneResponse?(dto: TDTO): TEntity;
+
+  /**
+   * Transform the response of a `getMany()` call.
+   * If not provided, falls back to mapping each item via `transformToEntity`.
+   */
+  transformGetManyResponse?(dtos: TDTO[]): TEntity[];
+
+  /**
+   * Transform the payload before a `create()` call.
+   * If not provided, falls back to `transformToDTO`.
+   */
+  transformCreatePayload?(entity: Partial<TEntity>): Partial<TDTO>;
+
+  /**
+   * Transform the payload before an `edit()` call.
+   * If not provided, falls back to `transformToDTO`.
+   */
+  transformEditPayload?(entity: Partial<TEntity>): Partial<TDTO>;
+}
+
+// ─── Abstract Base Transformer ──────────────────────────────────
+
+/**
+ * Abstract base class providing operation-specific transformation
+ * hooks with sensible passthrough defaults.
+ *
+ * Extends the core `IDataTransformer` contract with granular hooks
+ * for each CRUD operation. Override only the hooks you need —
+ * unoverridden hooks delegate to the core `transformToEntity` /
+ * `transformToDTO` methods.
+ *
+ * **Design rationale (from legacy analysis):**
+ * The legacy `BaseTransformer` used operation-specific methods
+ * (`transformerGetOne`, `transformerCreate`, etc.) because different
+ * operations often need different transformations. For example,
+ * `getOne` might need to compute derived fields, while `create`
+ * might need to strip IDs. This pattern is preserved here with
+ * proper typing.
+ *
+ * @typeParam TEntity - The frontend domain entity type
+ * @typeParam TDTO    - The API response/request DTO shape
+ *
+ * @example
+ * ```ts
+ * class BookingTransformer extends BaseDataTransformer<BookingEntity, BookingDTO> {
+ *   transformToEntity(dto: BookingDTO): BookingEntity {
+ *     return {
+ *       id: dto.id,
+ *       bookingCode: dto.booking_code,
+ *       customerName: dto.customer_name,
+ *     };
+ *   }
+ *
+ *   transformToDTO(entity: BookingEntity): BookingDTO {
+ *     return {
+ *       id: entity.id,
+ *       booking_code: entity.bookingCode,
+ *       customer_name: entity.customerName,
+ *     };
+ *   }
+ *
+ *   // Override only when getOne needs extra computed fields
+ *   transformGetOneResponse(dto: BookingDTO): BookingEntity {
+ *     const entity = this.transformToEntity(dto);
+ *     return { ...entity, durationNights: computeNights(dto) };
+ *   }
+ * }
+ * ```
+ */
+export abstract class BaseDataTransformer<
+  TEntity extends BaseEntity = BaseEntity,
+  TDTO = TEntity,
+> implements IDataTransformer<TEntity, TDTO>
+{
+  /**
+   * Core DTO → Entity transformation.
+   *
+   * Default implementation performs an identity cast.
+   * Override this in concrete transformers to provide
+   * the actual mapping logic.
+   *
+   * @param dto - Raw data from the API response
+   * @returns The mapped domain entity
+   */
+  transformToEntity(dto: TDTO): TEntity {
+    return dto as unknown as TEntity;
+  }
+
+  /**
+   * Core Entity → DTO transformation.
+   *
+   * Default implementation performs an identity cast.
+   * Override this in concrete transformers to provide
+   * the actual mapping logic.
+   *
+   * @param entity - Domain entity from the frontend
+   * @returns The mapped DTO for the API request
+   */
+  transformToDTO(entity: TEntity): TDTO {
+    return entity as unknown as TDTO;
+  }
+
+  // ─── Operation-Specific Hooks ───────────────────────────────
+
+  /**
+   * Transform the response of a `getOne()` call.
+   *
+   * Override this when `getOne` needs additional computed fields
+   * or different mapping than the default `transformToEntity`.
+   *
+   * @param dto - Single DTO from the API response
+   * @returns The mapped domain entity
+   */
+  transformGetOneResponse(dto: TDTO): TEntity {
+    return this.transformToEntity(dto);
+  }
+
+  /**
+   * Transform the response of a `getMany()` call.
+   *
+   * Override this when list responses need bulk transformations
+   * (e.g., sorting, filtering, or status normalization) beyond
+   * per-item mapping.
+   *
+   * @param dtos - Array of DTOs from the API response
+   * @returns Array of mapped domain entities
+   */
+  transformGetManyResponse(dtos: TDTO[]): TEntity[] {
+    return dtos.map((dto) => this.transformToEntity(dto));
+  }
+
+  /**
+   * Transform the payload before a `create()` call.
+   *
+   * Override this when create payloads need special handling
+   * (e.g., stripping IDs, formatting dates, converting nested
+   * structures).
+   *
+   * @param entity - Partial entity data from the frontend
+   * @returns The mapped partial DTO for the API request
+   */
+  transformCreatePayload(entity: Partial<TEntity>): Partial<TDTO> {
+    return this.transformToDTO(entity as TEntity) as Partial<TDTO>;
+  }
+
+  /**
+   * Transform the payload before an `edit()` call.
+   *
+   * Override this when edit payloads need special handling
+   * that differs from create (e.g., preserving certain
+   * read-only fields, handling delta updates).
+   *
+   * @param entity - Partial entity data from the frontend
+   * @returns The mapped partial DTO for the API request
+   */
+  transformEditPayload(entity: Partial<TEntity>): Partial<TDTO> {
+    return this.transformToDTO(entity as TEntity) as Partial<TDTO>;
+  }
+}
@@ -1,7 +1,9 @@
 import { describe, it, expect, vi, beforeEach } from 'vitest';
 import type { AxiosInstance, AxiosRequestConfig } from 'axios';
 import type { BaseEntity } from './types';
+import type { IDataTransformer } from './base-data.transformer';
 import { CommonRemoteDataServices } from './common-remote.data-services';
+import { BaseDataTransformer } from './base-data.transformer';

 // ─── Mock AxiosInstance ─────────────────────────────────────────

@@ -261,3 +263,398 @@ describe('BaseRemoteDataServices (via CommonRemoteDataServices)', () => {
    });
  });
 });
+
+// ═══════════════════════════════════════════════════════════════════
+// Data Transformer Integration Tests
+// ═══════════════════════════════════════════════════════════════════
+
+// ─── Test DTO (snake_case API shape) ────────────────────────────
+
+interface TestDTO {
+  id?: string;
+  booking_code: string;
+  customer_name: string;
+}
+
+interface TestEntity2 extends BaseEntity {
+  bookingCode: string;
+  customerName: string;
+}
+
+// ─── Concrete Transformer for Testing ───────────────────────────
+
+class TestTransformer extends BaseDataTransformer<TestEntity2, TestDTO> {
+  transformToEntity(dto: TestDTO): TestEntity2 {
+    return {
+      id: dto.id,
+      bookingCode: dto.booking_code,
+      customerName: dto.customer_name,
+    };
+  }
+
+  transformToDTO(entity: TestEntity2): TestDTO {
+    return {
+      id: entity.id,
+      booking_code: entity.bookingCode,
+      customer_name: entity.customerName,
+    };
+  }
+}
+
+// ─── Transformer with Custom Hooks ──────────────────────────────
+
+class CustomHookTransformer extends TestTransformer {
+  override transformGetOneResponse(dto: TestDTO): TestEntity2 {
+    const entity = this.transformToEntity(dto);
+    return { ...entity, customerName: entity.customerName.toUpperCase() };
+  }
+
+  override transformGetManyResponse(dtos: TestDTO[]): TestEntity2[] {
+    return dtos
+      .map((dto) => this.transformToEntity(dto))
+      .map((entity) => ({ ...entity, bookingCode: `LIST-${entity.bookingCode}` }));
+  }
+
+  override transformCreatePayload(entity: Partial<TestEntity2>): Partial<TestDTO> {
+    const dto = super.transformCreatePayload(entity);
+    return { ...dto, id: undefined };
+  }
+
+  override transformEditPayload(entity: Partial<TestEntity2>): Partial<TestDTO> {
+    const dto = super.transformEditPayload(entity);
+    return { ...dto, booking_code: `EDIT-${dto.booking_code}` };
+  }
+}
+
+// ─── Transformer Integration Tests ──────────────────────────────
+
+describe('BaseRemoteDataServices — Data Transformer Integration', () => {
+  let mockClient: AxiosInstance;
+
+  beforeEach(() => {
+    mockClient = createMockHttpClient();
+  });
+
+  // ── Without Transformer (backward compatibility) ──────────────
+
+  describe('without transformer (backward compatibility)', () => {
+    it('getOne() returns raw API response unchanged', async () => {
+      const rawDTO = { id: '42', booking_code: 'BK042', customer_name: 'Alice' };
+      (mockClient.request as ReturnType<typeof vi.fn>).mockResolvedValueOnce({
+        data: rawDTO,
+        status: 200,
+      });
+
+      const services = new CommonRemoteDataServices(mockClient, {
+        apiUrl: '/bookings',
+      });
+
+      const result = await services.getOne('42');
+      expect(result.data).toEqual(rawDTO);
+    });
+
+    it('getMany() returns raw API response unchanged', async () => {
+      const rawDTOs = [
+        { id: '1', booking_code: 'BK001', customer_name: 'Alice' },
+        { id: '2', booking_code: 'BK002', customer_name: 'Bob' },
+      ];
+      (mockClient.request as ReturnType<typeof vi.fn>).mockResolvedValueOnce({
+        data: rawDTOs,
+        status: 200,
+      });
+
+      const services = new CommonRemoteDataServices(mockClient, {
+        apiUrl: '/bookings',
+      });
+
+      const result = await services.getMany();
+      expect(result.data).toEqual(rawDTOs);
+    });
+
+    it('create() sends entity data as-is without transformation', async () => {
+      const services = new CommonRemoteDataServices(mockClient, {
+        apiUrl: '/bookings',
+      });
+
+      const entityData = { id: 'temp-1' };
+      await services.create(entityData);
+
+      const requestArg = (mockClient.request as ReturnType<typeof vi.fn>).mock.calls[0][0];
+      expect(requestArg.data).toEqual(entityData);
+    });
+  });
+
+  // ── With Base Transformer (core methods) ──────────────────────
+
+  describe('with transformer (core transformToEntity / transformToDTO)', () => {
+    let services: CommonRemoteDataServices<TestEntity2, TestDTO>;
+    const transformer = new TestTransformer();
+
+    beforeEach(() => {
+      services = new CommonRemoteDataServices<TestEntity2, TestDTO>(mockClient, {
+        apiUrl: '/bookings',
+        moduleKey: 'BOOKING',
+        transformer,
+      });
+    });
+
+    it('getOne() transforms API DTO to domain entity', async () => {
+      (mockClient.request as ReturnType<typeof vi.fn>).mockResolvedValueOnce({
+        data: { id: '42', booking_code: 'BK042', customer_name: 'Alice' },
+        status: 200,
+      });
+
+      const result = await services.getOne('42');
+
+      expect(result.data).toEqual({
+        id: '42',
+        bookingCode: 'BK042',
+        customerName: 'Alice',
+      });
+      expect(result.status).toBe(200);
+    });
+
+    it('getMany() transforms each DTO in the array to entities', async () => {
+      (mockClient.request as ReturnType<typeof vi.fn>).mockResolvedValueOnce({
+        data: [
+          { id: '1', booking_code: 'BK001', customer_name: 'Alice' },
+          { id: '2', booking_code: 'BK002', customer_name: 'Bob' },
+        ],
+        status: 200,
+      });
+
+      const result = await services.getMany();
+
+      expect(result.data).toEqual([
+        { id: '1', bookingCode: 'BK001', customerName: 'Alice' },
+        { id: '2', bookingCode: 'BK002', customerName: 'Bob' },
+      ]);
+    });
+
+    it('getMany() handles empty array response', async () => {
+      (mockClient.request as ReturnType<typeof vi.fn>).mockResolvedValueOnce({
+        data: [],
+        status: 200,
+      });
+
+      const result = await services.getMany();
+
+      expect(result.data).toEqual([]);
+    });
+
+    it('create() transforms entity payload to DTO before sending', async () => {
+      await services.create({
+        bookingCode: 'BK001',
+        customerName: 'Alice',
+      });
+
+      const requestArg = (mockClient.request as ReturnType<typeof vi.fn>).mock.calls[0][0];
+      expect(requestArg.data).toEqual({
+        id: undefined,
+        booking_code: 'BK001',
+        customer_name: 'Alice',
+      });
+    });
+
+    it('edit() transforms entity payload to DTO before sending', async () => {
+      await services.edit('42', {
+        bookingCode: 'BK042-UPDATED',
+        customerName: 'Bob',
+      });
+
+      const requestArg = (mockClient.request as ReturnType<typeof vi.fn>).mock.calls[0][0];
+      expect(requestArg.data).toEqual({
+        id: undefined,
+        booking_code: 'BK042-UPDATED',
+        customer_name: 'Bob',
+      });
+      expect(requestArg.url).toBe('/bookings/42');
+    });
+
+    it('delete() is unaffected by transformer (no data transformation needed)', async () => {
+      await services.delete('42');
+
+      expect(mockClient.request).toHaveBeenCalledWith(
+        expect.objectContaining({
+          url: '/bookings/42',
+          method: 'DELETE',
+        }),
+      );
+    });
+
+    it('customRequest() is unaffected by transformer', async () => {
+      await services.customRequest({
+        url: '/bookings/42/calculate-tax',
+        method: 'POST',
+        data: { items: [] },
+      });
+
+      const requestArg = (mockClient.request as ReturnType<typeof vi.fn>).mock.calls[0][0];
+      expect(requestArg.data).toEqual({ items: [] });
+    });
+  });
+
+  // ── With Custom Hook Transformer ──────────────────────────────
+
+  describe('with custom operation-specific hooks', () => {
+    let services: CommonRemoteDataServices<TestEntity2, TestDTO>;
+    const transformer = new CustomHookTransformer();
+
+    beforeEach(() => {
+      services = new CommonRemoteDataServices<TestEntity2, TestDTO>(mockClient, {
+        apiUrl: '/bookings',
+        moduleKey: 'BOOKING',
+        transformer,
+      });
+    });
+
+    it('getOne() uses transformGetOneResponse hook (uppercases customer name)', async () => {
+      (mockClient.request as ReturnType<typeof vi.fn>).mockResolvedValueOnce({
+        data: { id: '42', booking_code: 'BK042', customer_name: 'alice' },
+        status: 200,
+      });
+
+      const result = await services.getOne('42');
+
+      expect(result.data).toEqual({
+        id: '42',
+        bookingCode: 'BK042',
+        customerName: 'ALICE', // uppercased by custom hook
+      });
+    });
+
+    it('getMany() uses transformGetManyResponse hook (prefixes booking code)', async () => {
+      (mockClient.request as ReturnType<typeof vi.fn>).mockResolvedValueOnce({
+        data: [
+          { id: '1', booking_code: 'BK001', customer_name: 'Alice' },
+        ],
+        status: 200,
+      });
+
+      const result = await services.getMany();
+
+      expect(result.data).toEqual([
+        { id: '1', bookingCode: 'LIST-BK001', customerName: 'Alice' },
+      ]);
+    });
+
+    it('create() uses transformCreatePayload hook (strips id)', async () => {
+      await services.create({
+        id: 'should-be-removed',
+        bookingCode: 'BK001',
+        customerName: 'Alice',
+      });
+
+      const requestArg = (mockClient.request as ReturnType<typeof vi.fn>).mock.calls[0][0];
+      expect(requestArg.data.id).toBeUndefined();
+      expect(requestArg.data.booking_code).toBe('BK001');
+    });
+
+    it('edit() uses transformEditPayload hook (prefixes booking code)', async () => {
+      await services.edit('42', {
+        bookingCode: 'BK042',
+        customerName: 'Alice',
+      });
+
+      const requestArg = (mockClient.request as ReturnType<typeof vi.fn>).mock.calls[0][0];
+      expect(requestArg.data.booking_code).toBe('EDIT-BK042');
+    });
+  });
+
+  // ── Identity Transformer ──────────────────────────────────────
+
+  describe('with identity transformer (default passthrough)', () => {
+    it('produces same results as no transformer', async () => {
+      const rawData = { id: '1', name: 'Test' };
+      (mockClient.request as ReturnType<typeof vi.fn>).mockResolvedValue({
+        data: rawData,
+        status: 200,
+      });
+
+      // Service without transformer
+      const servicesNoTransformer = new CommonRemoteDataServices(mockClient, {
+        apiUrl: '/items',
+      });
+
+      // Service with identity transformer (no method overrides)
+      class IdentityTransformer extends BaseDataTransformer {}
+      const servicesWithIdentity = new CommonRemoteDataServices(mockClient, {
+        apiUrl: '/items',
+        transformer: new IdentityTransformer(),
+      });
+
+      const resultWithout = await servicesNoTransformer.getOne('1');
+      const resultWith = await servicesWithIdentity.getOne('1');
+
+      expect(resultWithout.data).toEqual(resultWith.data);
+    });
+  });
+
+  // ── Transformer receives correct arguments ────────────────────
+
+  describe('transformer method invocation', () => {
+    it('transformGetOneResponse receives the raw DTO from API', async () => {
+      const rawDTO = { id: '42', booking_code: 'BK042', customer_name: 'Alice' };
+      (mockClient.request as ReturnType<typeof vi.fn>).mockResolvedValueOnce({
+        data: rawDTO,
+        status: 200,
+      });
+
+      const mockTransformer: IDataTransformer<TestEntity2, TestDTO> = {
+        transformToEntity: vi.fn((dto: TestDTO) => ({
+          id: dto.id,
+          bookingCode: dto.booking_code,
+          customerName: dto.customer_name,
+        })),
+        transformToDTO: vi.fn(),
+        transformGetOneResponse: vi.fn((dto: TestDTO) => ({
+          id: dto.id,
+          bookingCode: dto.booking_code,
+          customerName: dto.customer_name,
+        })),
+        transformGetManyResponse: vi.fn(),
+        transformCreatePayload: vi.fn(),
+        transformEditPayload: vi.fn(),
+      };
+
+      const services = new CommonRemoteDataServices<TestEntity2, TestDTO>(mockClient, {
+        apiUrl: '/bookings',
+        transformer: mockTransformer,
+      });
+
+      await services.getOne('42');
+
+      expect(mockTransformer.transformGetOneResponse).toHaveBeenCalledWith(rawDTO);
+      expect(mockTransformer.transformToEntity).not.toHaveBeenCalled();
+    });
+
+    it('transformCreatePayload receives the entity data from caller', async () => {
+      const entityData: Partial<TestEntity2> = {
+        bookingCode: 'BK001',
+        customerName: 'Alice',
+      };
+
+      const mockTransformer: IDataTransformer<TestEntity2, TestDTO> = {
+        transformToEntity: vi.fn(),
+        transformToDTO: vi.fn(),
+        transformGetOneResponse: vi.fn(),
+        transformGetManyResponse: vi.fn(),
+        transformCreatePayload: vi.fn((entity) => ({
+          booking_code: entity.bookingCode!,
+          customer_name: entity.customerName!,
+        })),
+        transformEditPayload: vi.fn(),
+      };
+
+      const services = new CommonRemoteDataServices<TestEntity2, TestDTO>(mockClient, {
+        apiUrl: '/bookings',
+        transformer: mockTransformer,
+      });
+
+      await services.create(entityData);
+
+      expect(mockTransformer.transformCreatePayload).toHaveBeenCalledWith(entityData);
+      expect(mockTransformer.transformToDTO).not.toHaveBeenCalled();
+    });
+  });
+});
@@ -7,6 +7,7 @@ import type {
  ExecuteOptions,
  DataServicesConfig,
 } from './types';
+import type { IDataTransformer } from './base-data.transformer';
 import type { ApiResponse } from '../http-client/types';
 import { interpolateUrl } from './url-builder';
 import { DEFAULT_METHODS, DESCRIPTORS, makeDefaultURLs } from './constants';
@@ -26,9 +27,11 @@ import { DEFAULT_METHODS, DESCRIPTORS, makeDefaultURLs } from './constants';
 * - Includes `customRequest<T>()` as an escape hatch for non-standard endpoints
 *
 * @typeParam E    - The domain entity type (must extend BaseEntity)
+ * @typeParam TDTO - The API DTO shape (defaults to E for backward compatibility)
 *
 * @example
 * ```ts
+ * // Without transformer (backward compatible)
 * class BookingDataServices extends BaseRemoteDataServices<BookingEntity> {}
 *
 * const services = new BookingDataServices(apiClient, {
@@ -36,10 +39,20 @@ import { DEFAULT_METHODS, DESCRIPTORS, makeDefaultURLs } from './constants';
 *   moduleKey: 'BOOKING',
 * });
 *
+ * // With transformer (DTO ↔ Entity mapping)
+ * const services = new BookingDataServices(apiClient, {
+ *   apiUrl: '/bookings',
+ *   moduleKey: 'BOOKING',
+ *   transformer: new BookingTransformer(),
+ * });
+ *
 * const { data, status } = await services.getOne<BookingEntity>('42');
 * ```
 */
-export abstract class BaseRemoteDataServices<E extends BaseEntity = BaseEntity> {
+export abstract class BaseRemoteDataServices<
+  E extends BaseEntity = BaseEntity,
+  TDTO = E,
+> {
  /** The injected, isolated HTTP client instance. */
  protected readonly httpClient: AxiosInstance;

@@ -52,9 +65,18 @@ export abstract class BaseRemoteDataServices<E extends BaseEntity = BaseEntity>
  /** Module key for the 'ex-module-key' audit header. */
  protected readonly moduleKey: string | undefined;

-  constructor(httpClient: AxiosInstance, config: DataServicesConfig) {
+  /**
+   * Optional data transformer for DTO ↔ Entity mapping.
+   *
+   * When present, CRUD methods automatically transform responses
+   * and payloads. When absent, data passes through unchanged.
+   */
+  protected readonly transformer: IDataTransformer<E, TDTO> | undefined;
+
+  constructor(httpClient: AxiosInstance, config: DataServicesConfig<E, TDTO>) {
    this.httpClient = httpClient;
    this.moduleKey = config.moduleKey;
+    this.transformer = config.transformer;

    this.urls = {
      ...makeDefaultURLs(config.apiUrl ?? ''),
@@ -143,31 +165,85 @@ export abstract class BaseRemoteDataServices<E extends BaseEntity = BaseEntity>

  // ─── CRUD Operations ───────────────────────────────────────────

-  /** Fetch a paginated list of entities. */
-  getMany<T = E[]>(config?: AxiosRequestConfig): Promise<ApiResponse<T>> {
-    return this.execute<T>(DESCRIPTORS.getMany, { config });
+  /**
+   * Fetch a paginated list of entities.
+   *
+   * When a transformer is injected, the raw API response is passed
+   * through `transformGetManyResponse()` before being returned.
+   */
+  async getMany<T = E[]>(config?: AxiosRequestConfig): Promise<ApiResponse<T>> {
+    const result = await this.execute<T>(DESCRIPTORS.getMany, { config });
+
+    if (this.transformer && Array.isArray(result.data)) {
+      return {
+        ...result,
+        data: this.transformer.transformGetManyResponse
+          ? this.transformer.transformGetManyResponse(result.data as unknown as TDTO[])
+          : result.data.map((item: unknown) => this.transformer!.transformToEntity(item as TDTO)),
+      } as unknown as ApiResponse<T>;
    }

-  /** Fetch a single entity by ID. */
-  getOne<T = E>(id: string, config?: AxiosRequestConfig): Promise<ApiResponse<T>> {
-    return this.execute<T>(DESCRIPTORS.getOne, {
+    return result;
+  }
+
+  /**
+   * Fetch a single entity by ID.
+   *
+   * When a transformer is injected, the raw API response is passed
+   * through `transformGetOneResponse()` before being returned.
+   */
+  async getOne<T = E>(id: string, config?: AxiosRequestConfig): Promise<ApiResponse<T>> {
+    const result = await this.execute<T>(DESCRIPTORS.getOne, {
      variableURL: { id },
      config,
    });
+
+    if (this.transformer && result.data != null) {
+      return {
+        ...result,
+        data: this.transformer.transformGetOneResponse
+          ? this.transformer.transformGetOneResponse(result.data as unknown as TDTO)
+          : this.transformer.transformToEntity(result.data as unknown as TDTO),
+      } as unknown as ApiResponse<T>;
    }

-  /** Create a new entity. */
+    return result;
+  }
+
+  /**
+   * Create a new entity.
+   *
+   * When a transformer is injected, the entity payload is passed
+   * through `transformCreatePayload()` before being sent to the API.
+   */
  create<T = E>(data: Partial<E>, config?: AxiosRequestConfig): Promise<ApiResponse<T>> {
+    const transformedData = this.transformer?.transformCreatePayload
+      ? this.transformer.transformCreatePayload(data)
+      : this.transformer
+        ? this.transformer.transformToDTO(data as E)
+        : data;
+
    return this.execute<T>(DESCRIPTORS.create, {
-      config: { ...config, data },
+      config: { ...config, data: transformedData },
    });
  }

-  /** Update an existing entity by ID. */
+  /**
+   * Update an existing entity by ID.
+   *
+   * When a transformer is injected, the entity payload is passed
+   * through `transformEditPayload()` before being sent to the API.
+   */
  edit<T = E>(id: string, data: Partial<E>, config?: AxiosRequestConfig): Promise<ApiResponse<T>> {
+    const transformedData = this.transformer?.transformEditPayload
+      ? this.transformer.transformEditPayload(data)
+      : this.transformer
+        ? this.transformer.transformToDTO(data as E)
+        : data;
+
    return this.execute<T>(DESCRIPTORS.edit, {
      variableURL: { id },
-      config: { ...config, data },
+      config: { ...config, data: transformedData },
    });
  }

@@ -13,10 +13,11 @@ import { BaseRemoteDataServices } from './base-remote.data-services';
 * custom methods using `this.execute()` or `this.customRequest()`.
 *
 * @typeParam E    - The domain entity type
+ * @typeParam TDTO - The API DTO shape (defaults to E for backward compatibility)
 *
 * @example
 * ```ts
- * // Direct instantiation for standard modules
+ * // Direct instantiation for standard modules (no transformer)
 * const bookingServices = new CommonRemoteDataServices<BookingEntity>(
 *   apiClient,
 *   { apiUrl: '/bookings', moduleKey: 'BOOKING' },
@@ -27,6 +28,19 @@ import { BaseRemoteDataServices } from './base-remote.data-services';
 *
 * @example
 * ```ts
+ * // With transformer for DTO ↔ Entity mapping
+ * const bookingServices = new CommonRemoteDataServices<BookingEntity, BookingDTO>(
+ *   apiClient,
+ *   {
+ *     apiUrl: '/bookings',
+ *     moduleKey: 'BOOKING',
+ *     transformer: new BookingTransformer(),
+ *   },
+ * );
+ * ```
+ *
+ * @example
+ * ```ts
 * // For modules needing custom operations, extend the base:
 * class InvoiceDataServices extends BaseRemoteDataServices<InvoiceEntity> {
 *   async calculateTax(invoiceId: string) {
@@ -40,4 +54,6 @@ import { BaseRemoteDataServices } from './base-remote.data-services';
 */
 export class CommonRemoteDataServices<
  E extends BaseEntity = BaseEntity,
-> extends BaseRemoteDataServices<E> {}
+  TDTO = E,
+> extends BaseRemoteDataServices<E, TDTO> {}
+
@@ -1,6 +1,7 @@
 // ─── Classes ────────────────────────────────────────────────────
 export { BaseRemoteDataServices } from './base-remote.data-services';
 export { CommonRemoteDataServices } from './common-remote.data-services';
+export { BaseDataTransformer } from './base-data.transformer';

 // ─── Utilities ──────────────────────────────────────────────────
 export { interpolateUrl } from './url-builder';
@@ -17,3 +18,5 @@ export type {
  ExecuteOptions,
  DataServicesConfig,
 } from './types';
+
+export type { IDataTransformer } from './base-data.transformer';
@@ -1,5 +1,6 @@
 import type { AxiosRequestConfig } from 'axios';
 import type { TelemetryContext } from '../http-client/types';
+import type { IDataTransformer } from './base-data.transformer';

 // ─── Base Entity ────────────────────────────────────────────────

@@ -112,8 +113,14 @@ export interface ExecuteOptions {

 /**
 * Configuration for constructing a BaseRemoteDataServices instance.
+ *
+ * @typeParam TEntity - The frontend domain entity type (defaults to BaseEntity)
+ * @typeParam TDTO    - The API DTO shape (defaults to TEntity for backward compatibility)
 */
-export interface DataServicesConfig {
+export interface DataServicesConfig<
+  TEntity extends BaseEntity = BaseEntity,
+  TDTO = TEntity,
+> {
  /** Base API path (e.g., '/bookings'). Used to generate all URL templates. */
  apiUrl?: string;
  /** Module key for the 'ex-module-key' header (e.g., 'BOOKING'). */
@@ -122,4 +129,18 @@ export interface DataServicesConfig {
  urls?: Partial<ApiURLMap>;
  /** Override specific HTTP methods. */
  methods?: Partial<RequestMethodMap>;
+
+  /**
+   * Optional data transformer for DTO ↔ Entity mapping.
+   *
+   * When provided, CRUD methods automatically transform:
+   * - **Responses** (`getOne`, `getMany`): DTO → Entity via transformer
+   * - **Payloads** (`create`, `edit`): Entity → DTO via transformer
+   *
+   * When omitted, data passes through unchanged (backward compatible).
+   *
+   * @see {@link IDataTransformer} for the transformer contract
+   * @see {@link BaseDataTransformer} for the abstract base class
+   */
+  transformer?: IDataTransformer<TEntity, TDTO>;
 }