feat(mongo): allow reusing mongo client via driverOptions

Closes #3352

packages/mongodb/src/MongoConnection.ts

@@ -30,9 +30,16 @@ export class MongoConnection extends Connection {
   }
 
   async connect(): Promise<void> {
-    this.client = new MongoClient(this.config.getClientUrl(), this.getConnectionOptions());
+    const driverOptions = this.config.get('driverOptions');
+
+    if (driverOptions instanceof MongoClient) {
+      this.logger.log('info', 'Reusing MongoClient provided via `driverOptions`');
+      this.client = driverOptions;
+    } else {
+      this.client = new MongoClient(this.config.getClientUrl(), this.getConnectionOptions());
+      await this.client.connect();
+    }
 
-    await this.client.connect();
     this.db = this.client.db(this.config.get('dbName'));
     this.connected = true;
   }
@@ -46,6 +53,10 @@ export class MongoConnection extends Connection {
     return this.connected;
   }
 
+  getClient(): MongoClient {
+    return this.client;
+  }
+
   getCollection<T>(name: EntityName<T>): Collection<T> {
     return this.db.collection<T>(this.getCollectionName(name));
   }

tests/features/reusing-mongo-client.test.ts

@@ -0,0 +1,24 @@
+import { MikroORM } from '@mikro-orm/core';
+import { MongoDriver } from '@mikro-orm/mongodb';
+import { Author, schema } from '../entities';
+
+test('should allow reusing mongo connection', async () => {
+  const orm = await MikroORM.init({
+    driver: MongoDriver,
+    dbName: 'mikro_orm_test',
+    entities: [Author, schema],
+  });
+  const mongo = orm.em.getConnection().getClient();
+
+  const orm2 = await MikroORM.init({
+    driver: MongoDriver,
+    dbName: 'mikro_orm_test',
+    entities: [Author, schema],
+    driverOptions: mongo,
+  });
+
+  await expect(orm.em.find(Author, {})).resolves.toHaveLength(0);
+  await orm2.close(); // closing orm2 will make orm1 disconnect too as they share mongo client
+  await expect(orm.isConnected()).resolves.toBe(true);
+  await expect(orm.em.find(Author, {})).rejects.toThrow('Client must be connected before running operations');
+});