Rocket.Chat/server/startup/migrations/v011.js

RocketChat.Migrations.add({
	version: 11,
	up() {
		/*
		 * Set GENERAL room to be default
		 */
		RocketChat.models.Rooms.update({
			_id: 'GENERAL'
		}, {
			$set: {
				'default': true
			}
		});

		return console.log('Set GENERAL room to be default');
	}
});