class DbConnection {
  constructor() {
    this._con = undefined;
    this._pool = undefined;
  }

  _createPool(  database , user, password ) {
    this._pool =  mysql.createPool({
      connectionLimit: 100,
      host: "localhost",
      port: 3306,
      database: database,
      user: user,
      password: password
    });
  }

  async openConnection( req ) {
    this._createPool(req.database, req.user, req.password );

    return new Promise((resolve, reject) => {
      this._pool.getConnection((err, connection) => {
        if (err)
          reject( err );
        this._con = connection;
        resolve();
      });
    });
  }

  async beginTransaction() {
    return new Promise((resolve, reject) => {
      this.con.beginTransaction(err => {
        if (err) {
          this.con.rollback( () => this.con.release() );
          reject( err );
        }
        resolve();
      });
    });
  }

  get con() {
    return this._con;
  }

  close() {
    if (this.con)
      this.con.release();
  }
}

class GenericCtrl {

  constructor( db, dao ) {
    this._db = db;
    this._dao = dao;
  }

  async buscar( req, res ) {
    req.assert( 'id', 'um id é necessário.' ).notEmpty().isInt().withMessage('Deve ser inteiro');

    if (req.validationErrors())
      return res.status( 400 ).json( { erros: req.validationErrors() } );

    try {
      await this._db.openConnection( req );
      let response = await this._dao.read( req.params.id  );

      res.status( 200 ).json( response );

    } catch (e) {
      res.status( 500 ).json( { erro: e.toString() } );

    } finally {
      this._db.close();
    }
  }

}

class GenericDao {

  constructor( db ) {
    this._db = db;
  }

  get con() {
    return this._db.con;
  }

  read( query, sql = undefined, inserts = undefined ) {

    sql = "SELECT * FROM ?? WHERE ?? = ?";
    inserts = [ 'Usuario', 'id', 23 ];

    sql = mysql.format(sql, inserts);

    return new Promise( ( resolve, reject ) => {
      this.con.query( sql, ( err, result ) => err ? reject( err ) : resolve( result ) );
    });
  }

}