Discussion on: Advanced NestJS: How to build completely dynamic NestJS modules

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Wow, that's a lot of code. And what is the benefit of this? In my opinion, it is better to transfer the metadata for the database connection synchronously:

import { ConnectionConfig, TypeCast } from 'mysql';

const {
  MYSQL_HOST,
  MYSQL_PORT,
  MYSQL_USER,
  MYSQL_PASSWORD,
  MYSQL_DATABASE,
  MYSQL_CHARSET
} = process.env;

export class MySqlConfigService implements ConnectionConfig {
  charset = MYSQL_CHARSET;
  host = MYSQL_HOST;
  port = MYSQL_PORT ? +MYSQL_PORT : 3306;
  user = MYSQL_USER;
  password = MYSQL_PASSWORD;
  database = MYSQL_DATABASE;
  typeCast: TypeCast = (field, next) => {
    if (field.type == 'JSON') {
      return JSON.parse(`${field.string()}` || '');
    }
    return next();
  };
}

Pass this config to DI:

providers: [
  MySqlConfigService
  //...
]

And when it's time to execute requests to the database, then make an asynchronous connection to the database. This approach allows, in addition to simplifying the code, to reconnect to a database when needed:

import { createPool, Pool, PoolConnection, MysqlError, OkPacket } from 'mysql';
import { Injectable } from '@nestjs/common';

import { MySqlConfigService } from './mysql-config.service';

@Injectable()
export class MysqlService {
  private pools: { [database: string]: Pool } = {};

  constructor(
    private config: MySqlConfigService,
    // ...
  ) {}

  getConnection(dbName?: string): Promise<PoolConnection> {
    return new Promise((resolve, reject) => {
      const config = { ...this.config };
      const database = (dbName || config.database) as string;
      config.database = database;
      if (!this.pools[database]) {
        this.pools[database] = createPool(config);
      }

      this.pools[database].getConnection((err, connection) => {
        if (err) {
          this.handleErr(this.serverMsg.mysqlConnect, err, reject);
        } else {
          resolve(connection);
        }
      });
    });
  }
  // ...
}