NestJS + MySQL: how to connect to multiple databases without setting entities - mysql

Question

The NestJS documentation about Databases explains how to connect to MySQL using TypeORM and defining Entities.
In my case, I have to connect to an MySQL server that has more than one database and I need to execute SQL queries directly (without using the Entity layer) and fetch their results. Sometimes I also need to run cross-database queries.
How to do that using NestJS?

Answer 1

You can import multiple databases by given the connections different names. You can either directly pass the database configuration in separate TypeOrmModule.forRoot({...}) imports or use a ormconfig.json config file. (However, the ormconfig.json file might still not work with multiple databases, see this thread.)
TypeOrmModule.forRoot({
...defaultOptions,
name: 'personsConnection',
^^^^^^^^^^^^^^^^^^^^^^^^^^
host: 'person_db_host',
entities: [Person],
}),
TypeOrmModule.forRoot({
...defaultOptions,
name: 'albumsConnection',
^^^^^^^^^^^^^^^^^^^^^^^^^
host: 'album_db_host',
// You can also leave the entities empty
entities: [],
})
As stated by Kamil in the comments, you can inject the TypeORM connection object with #InjectConnection('albumsConnection'): Connection and then use the QueryBuilder or the method query to run raw SQL.
const rawData = await connection.query(`SELECT * FROM USERS`);