Laravel Migrations: nullable and constrained - in which order?

When creating the foreign key migrations in Laravel, a short syntax is constrained(). With that, it's easy to make one mistake related to Column modifiers.

If you want that foreign key column to be nullable, which syntax is correct?

// Option A:
$table->foreignId('user_id')->nullable()->constrained();
 
// Or, Option B:
$table->foreignId('user_id')->constrained()->nullable();

Perhaps, you would say both?

Wrong.

On the official documentation page for the foreign keys, you may find this important sentence:

Any additional column modifiers must be called before the constrained method.

Yes, nullable() is one of the so-called column modifiers. So yeah, the correct way is nullable before constrained.

But wait, what happens if we put it the other way around?

Schema::create('tasks', function (Blueprint $table) {
    $table->id();
    $table->string('name');
    $table->foreignId('user_id')->constrained()->nullable(); //INCORRECT!
    $table->timestamps();
});

Would it throw an error? Not quite.

In fact, the migration will actually succeed.

That's what makes this mistake so dangerous: you may not even notice it.

You will have an issue only when saving the data:

Task::create(['name' => 'Testing']);
 
// Will throw this:
Illuminate\Database\QueryException with message 'SQLSTATE[HY000]:
General error: 1364 Field 'user_id' doesn't have a default value
    (SQL: insert into `tasks` (`name`, `updated_at`, `created_at`)
        values (Testing, 2022-10-05 11:48:05, 2022-10-05 11:48:05))'

So, be careful and don't forget to add nullable before constrained.

In fact, it's also applicable to any other column modifiers. A few typical examples:

• ->after('name')->constrained(); and not ->constrained()->after('name');
• ->default(1)->constrained(); and not ->constrained()->default(1);
• ->comment('Some comment')->constrained(); and not ->constrained()->comment('Some comment');