Why is ForeignKey not added in migrations?

Question

MySQL

I am migrating to MySQL in yii2

Migration code:

$this->addColumn('{{%trainer}}', 'studio_id', $this->integer()->notNull());
$this->addForeignKey('fk-trainer-studio', '{{%trainer}}', 'studio_id', '{{%studio}}', 'id', 'CASCADE', 'RESTRICT');

Swears on data integrity. There is a `studio` table, in general I took a similar code from another migration

Mistake:

add foreign key fk-trainer-studio: {{%trainer}} (studio_id) references {{%studio}} (id) ...Exception: 
SQLSTATE[23000]: Integrity constraint violation: 1452 
Cannot add or update a child row: a foreign key constraint fails 
(`db`.`#sql-6cb_105`, CONSTRAINT `fk-trainer-studio` FOREIGN KEY (`studio_id`) REFERENCES `studio` (`id`) ON DELETE CASCADE)

Answer 1

most likely in the table to which you want to add a foreign key there is data that refers to non-existent records