protected $_constraints = [
		'primary' => ['template' => 'PRIMARY KEY ({:column})'],
		'foreign_key' => [
			'template' => 'FOREIGN KEY ({:column}) REFERENCES {:to} ({:toColumn}) {:on}'
		],
		'unique' => [
			'template' => 'UNIQUE {:index} ({:column})'
		],
		'check' => ['template' => 'CHECK ({:expr})']
	];