# OverrideModule

**Kind:** Interface

**Source:** [`packages/testing/interfaces/override-module.interface.ts`](https://github.com/nestjs/nest/blob/master/packages/testing/interfaces/override-module.interface.ts#L7)

**Part of:** [Testing](subsystem-packages-testing)

`OverrideModule` defines the fluent API used to replace an existing module definition while building a test module. Calling `useModule` supplies a new `ModuleDefinition` and returns the `TestingModuleBuilder` so additional test configuration can be chained.

## Properties

| Property | Type |
|---|---|
| `useModule` | `(newModule: ModuleDefinition) => TestingModuleBuilder` |

## Diagram

```mermaid
graph LR
  A[TestingModuleBuilder] -->|overrideModule(targetModule)| B[OverrideModule]
  B -->|useModule(newModule)| C[TestingModuleBuilder]
  C -->|compile()| D[TestingModule]
```

## Usage

```ts
import { Test } from '@nestjs/testing';
import { AppModule } from './app.module';
import { DatabaseModule } from './database.module';
import { InMemoryDatabaseModule } from './testing/in-memory-database.module';

const testingModule = await Test.createTestingModule({
  imports: [AppModule],
})
  .overrideModule(DatabaseModule)
  .useModule(InMemoryDatabaseModule)
  .compile();
```

## AI Coding Instructions

- Use `overrideModule()` before calling `useModule()`; `useModule()` is available on the returned `OverrideModule` interface.
- Pass a valid Nest `ModuleDefinition`, such as a module class or supported dynamic module definition.
- Preserve the fluent builder chain by using the `TestingModuleBuilder` returned from `useModule()`.
- Use module overrides for integration boundaries such as databases, messaging clients, or external-service modules.
- Ensure replacement modules provide any providers, exports, or imports required by modules under test.

## Relationships

- IMPORTS → `ModuleDefinition`