function onRead( error, db ) {
		var keys;
		var i;
		if ( error ) {
			return cb( error );
		}
		keys = objectKeys( db );
		for ( i = 0; i < keys.length; i++ ) {
			if ( db[ keys[ i ] ].id === id ) {
				return cb( null, keys[ i ] );
			}
		}
		cb( null, null );
	}
Callback invoked upon reading a file.

@private
@param {(Error|null)} error - error object
@param {Object} db - database
@returns {void}