Phactory\Sql\Association\ManyToOne::_guessToColumn PHP Method

    protected function _guessToColumn()
    {
        if (null === $this->_to_column) {
            $this->_to_column = $this->_to_table->getPrimaryKey();
            if (!$this->_to_column) {
                throw new \Exception("Unable to determine primary key for table '{$this->_to_table}' and none specified");
            }
        }
    }