protected function checkField( $field, $mandatory = FALSE, $strict = TRUE ){
		if( !is_string( $field ) ){
			if( !$strict )
				return FALSE;
			throw new \InvalidArgumentException( 'Field must be a string' );
		}
		$field	= trim( $field );
		if( !strlen( $field ) ){
			if( $mandatory ){
				if( !$strict )
					return FALSE;
				throw new \InvalidArgumentException( 'Field must have a value' );
			}
			return NULL;
		}
		if( !in_array( $field, $this->columns ) ){
			if( !$strict )
				return FALSE;
			$message	= 'Field "%s" is not an existing column of table %s';
			throw new \InvalidArgumentException( sprintf( $message, $field, $this->getName() ) );
		}
		return $field;
	}