Yii2迁移中的多对多关系

use yii\db\Migration;

class m200101_000000_create_relationship_table extends Migration
{
    public function up()
    {
        $this->createTable('relationship', [
            'entity1_id' => $this->integer(),
            'entity2_id' => $this->integer(),
        ]);

        $this->addPrimaryKey('pk_relationship', 'relationship', ['entity1_id', 'entity2_id']);

        $this->addForeignKey('fk_relationship_entity1', 'relationship', 'entity1_id', 'entity1', 'id', 'CASCADE', 'CASCADE');
        $this->addForeignKey('fk_relationship_entity2', 'relationship', 'entity2_id', 'entity2', 'id', 'CASCADE', 'CASCADE');
    }

    public function down()
    {
        $this->dropForeignKey('fk_relationship_entity1', 'relationship');
        $this->dropForeignKey('fk_relationship_entity2', 'relationship');

        $this->dropTable('relationship');
    }
}