laravel errno 150 foreign key constraint is incorrectly formed

Question

Can anybody help me to solve this problem?

There are 3 tables with 2 foreign keys:

Schema::create(\'users\', function (Blueprint $table) {
    $table-&         


        

Answer 1

Well this answer is related to Laravel 7.x. So the error:

errno: 150 "Foreign key constraint is incorrectly formed"

can occur due many reason while migrating the migrations. The one common reason that I am familiar about is order of migration.

Lets say we have two table "users" and "roles", and "users" table have a foreign key referring the "id" column on "roles" table. So make sure that "roles" is migrated before "users" table.

So order of migration is important. Its obvious as it does not make sense for MySQL to refer to "id" column of unknown table.

Second reason is wrong data type. In laravel 7.x we use "id()" method for primary key. So make sure that the intended foreign key (in my case "role_id" in "users" table) is of "bigInteger" and is "unsigned".

Here my code:

Schema::create('roles', function (Blueprint $table) {
        $table->id();
        $table->string('name');
        $table->string('slug')->nullable();
        $table->timestamps();
    });


Schema::create('users', function (Blueprint $table) {
        $table->id();
        $table->bigInteger("role_id")->unsigned();
        $table->string('name');
        $table->string('email')->unique();
        $table->timestamp('email_verified_at')->nullable();
        $table->string('password');
        $table->rememberToken();
        $table->timestamps();

        $table->foreign('role_id')->references('id')->on('roles')->onDelete('cascade');
    });

public function down()
{
    Schema::table('users', function(Blueprint $table)
    {
        $table->dropForeign('users_role_id_foreign'); 
    });
    Schema::dropIfExists('users');
}

So in the above code I had to migrate the "roles" table first then the "users" table. So MySQL can create foreign key for the roles table.

What is do is I move the child migration (migration having foreign key) to temporary folder. And restore it after migrating parent migration (in my case "roles" table and then migrate the child migration ("users" migration).

And as side tip: while dropping the dependent migration (migration containing foreign key) first drop the foreign key first. And Laravel uses specific naming convention while dropping foreign key "__foreign".

So happy coding and be ready for Ground breaking release of PHP 8.