public function push( Job $job, $delay = 0 ) {
		$result = $this->database->insert( $this->jobs_table, array(
			'job'          => serialize( $job ),
			'available_at' => $this->datetime( $delay ),
			'created_at'   => $this->datetime(),
		) );

		if ( ! $result ) {
			return false;
		}

		return $this->database->insert_id;
	}