public function schema($field = null) {
		$schema = array();

		switch (true) {
			case ($this->_schema):
				$schema = $this->_schema;
			break;
			case ($model = $this->_model):
				$schema = $model::schema();
			break;
		}
		if ($field) {
			return isset($schema[$field]) ? $schema[$field] : null;
		}
		return $schema;
	}