1215 无法在 laravel 中添加外键约束

Question

Schema::create('students', function (Blueprint $table) {
        $table->id();
        $table->integer('age')->unsigned();
        $table->integer('city_id')->unsigned();
        $table->foreign('city_id')->references('id')->on('cities')->onDelete('cascade');
        $table->timestamps();
});
Schema::create('cities', function (Blueprint $table) {
        $table->id();
        $table->text('name');
        $table->timestamps();
});

i'm new laravel, when i run migrate, in terminal show

SQLSTATE[HY000]: General error: 1215 Cannot add foreign key constraint (SQL: alter table students add constraint students_city_id_foreign foreign key ( city_id ) references cities ( id ) on delete cascade)

help me fix

Answer 1

Your problem is on the order of the code. You want to make a foreign key to a table which is not yet exists.

Try this instead:

Schema::create('students', function (Blueprint $table) {
        $table->id();
        $table->integer('age')->unsigned();
        $table->integer('city_id')->unsigned();
        $table->timestamps();
});

Schema::create('cities', function (Blueprint $table) {
        $table->id();
        $table->text('name');
        $table->timestamps();
});

Schema::table('students', function(Blueprint $table) {
        $table->foreign('city_id')->references('id')->on('cities')->onDelete('cascade');
    });

Like this, you first creates both tables, and after that you set the foreign key.

Answer 2

If you are using more recent version of Laravel id fields are now unsignedBigInteger field. So you should do something like:

Schema::create('cities', function (Blueprint $table) {
        $table->id();
        $table->text('name');
        $table->timestamps();
});
Schema::create('students', function (Blueprint $table) {
        $table->id();
        $table->integer('age')->unsigned();
        $table->foreignId('city_id')->constrained()->onDelete('cascade');
        $table->timestamps();
});

Also, make sure the cities migration is running before students.

Answer 3

=>your cities migration first create then after students migration create because your city_id foreign key reference to the cities table laravel migration check order and order wise migration run so first cities migration run(cities table create) then students migration run.