/**
 * Helper: Provides that only meant database fields are changed.
 *
 * @param  array              $fields   Fields (key, value pairs)
 * @param  array              $expected Changeable fields (just keys)
 * @throws HTTP_Exception_400           If any key of 1st array is not found in 2nd array, but is table column
 */
function check_fields($fields, $changeable) {

	$field_keys = array_keys($fields);
	$column_keys = array_keys($this->_data);

	foreach ($field_keys as $key) {

		if (is_array($fields[$key])) {

			$this->check_fields($fields[$key], $changeable[$key]);

		}

		if (!in_array($key, $changeable) && in_array($key, $column_keys)) {

			throw new HTTP_Exception_400("Trying to change database fields that aren't meant to be changed!");

		}

	}

}