Multiple database connections using getConnectionToken

Author: justvamp

content/techniques/mongo.md

@@ -222,6 +222,23 @@ export class CatsService {
 }
 ```
 
+It's also possible to inject any `Connection` to the providers:
+
+```typescript
+@Module({
+  providers: [
+    {
+      provide: CatsService,
+      useFactory: (catsConnection: Connection) => {
+        return new CatsService(catsConnection);
+      },
+      inject: [getConnectionToken('cats')],
+    },
+  ],
+})
+export class CatsModule {}
+```
+
 #### Hooks (middleware)
 
 Middleware (also called pre and post hooks) are functions which are passed control during execution of asynchronous functions. Middleware is specified on the schema level and is useful for writing plugins ([source](https://mongoosejs.com/docs/middleware.html)). Calling `pre()` or `post()` after compiling a model does not work in Mongoose. To register a hook **before** model registration, use the `forFeatureAsync()` method of the `MongooseModule` along with a factory provider (i.e., `useFactory`). With this technique, you can access a schema object, then use the `pre()` or `post()` method to register a hook on that schema. See example below:

content/techniques/sql.md

@@ -591,6 +591,23 @@ export class AlbumsService {
 }
 ```
 
+It's also possible to inject any `Connection` to the providers:
+
+```typescript
+@Module({
+  providers: [
+    {
+      provide: AlbumsService,
+      useFactory: (albumsConnection: Connection) => {
+        return new AlbumsService(albumsConnection);
+      },
+      inject: [getConnectionToken('albumsConnection')],
+    },
+  ],
+})
+export class AlbumsModule {}
+```
+
 #### Testing
 
 When it comes to unit testing an application, we usually want to avoid making a database connection, keeping our test suites independent and their execution process as fast as possible. But our classes might depend on repositories that are pulled from the connection instance. How do we handle that? The solution is to create mock repositories. In order to achieve that, we set up [custom providers](/fundamentals/custom-providers). Each registered repository is automatically represented by an `<EntityName>Repository` token, where `EntityName` is the name of your entity class.
@@ -1160,6 +1177,23 @@ export class AlbumsService {
 }
 ```
 
+It's also possible to inject any `Sequelize` instance to the providers:
+
+```typescript
+@Module({
+  providers: [
+    {
+      provide: AlbumsService,
+      useFactory: (albumsSequelize: Sequelize) => {
+        return new AlbumsService(albumsSequelize);
+      },
+      inject: [getConnectionToken('albumsConnection')],
+    },
+  ],
+})
+export class AlbumsModule {}
+```
+
 #### Testing
 
 When it comes to unit testing an application, we usually want to avoid making a database connection, keeping our test suites independent and their execution process as fast as possible. But our classes might depend on models that are pulled from the connection instance. How do we handle that? The solution is to create mock models. In order to achieve that, we set up [custom providers](/fundamentals/custom-providers). Each registered model is automatically represented by a `<ModelName>Model` token, where `ModelName` is the name of your model class.