src/Query.php
public function expr($properties = [], $arguments = null)
    {
        $c = $this->expression_class;
        $e = new $c($properties, $arguments);
        $e->connection = $this->connection;

        return $e;
    }
public function expr($properties = [], $arguments = null)
    {
        $c = $this->expression_class;
        $e = new $c($properties, $arguments);
        $e->connection = $this->connection;

        return $e;
    }
Returns Expression object for the corresponding Query
sub-class (e.g. Query_MySQL will return Expression_MySQL).

Connection is not mandatory, but if set, will be preserved. This
method should be used for building parts of the query internally.

@param array $properties
@param array $arguments

@return Expression