Laravel migration: “Foreign key constraint is incorrectly formed” (errno 150)

Question

When migrating my DB, this error appears. Below is my code followed by the error that I am getting when trying to run the migration.

Code

Answer 1

One way to get around foreign key errors is to disable checking: "SET FOREIGN_KEY_CHECKS". This is a palliative solution, but the correct thing is really to adjust the tables and their relationships.

   DB::statement('SET FOREIGN_KEY_CHECKS=0;');

   Schema::table('example', function (Blueprint $table) {

        $table->integer('fk_example')->unsigned()->index();
        $table->foreign('fk_example')->references('id')->on('examples');
    });

  DB::statement('SET FOREIGN_KEY_CHECKS=1;');

Answer 2

Laravel 5.8

In the foreign key column use unsignedBigInteger to avoid mismatch foreign key data type problem . For example let us assume we have two tables questions and replies
Questions table will look:

 public function up()
    {
        Schema::create('questions', function (Blueprint $table) {
            $table->bigIncrements('id');
             $table->text('body');
             $table->integer('user_id')->unsigned();
            $table->timestamps();
        });
    }

Replies table look like :

public function up()
{
    Schema::create('replies', function (Blueprint $table) {
        $table->bigIncrements('id');
        $table->text('body');
        $table->unsignedBigInteger('question_id');
        $table->integer('user_id')->unsigned();
        $table->foreign('question_id')->references('id')->on('questions')->onDelete('cascade');
        $table->timestamps();
    });
}

Answer 3

I had to face the same problem at Larvel 6. I solve this following way.

I think it helps you or others:

$table->increments('id');
    $table->bigInteger('user_id')->unsigned(); //chnage this line
    $table->bigInteger('category_id')->unsigned(); //change this line
    ---
    $table->foreign('user_id')
        ->references('id')
        ->on('users')
        ->onDelete('cascade');
    $table->foreign('category_id')
        ->references('id')
        ->on('categories')
        ->onDelete('cascade');

Incrementing ID using a "big integer" equivalent.

used bigInteger instead of Integer

2. If still now you got an error.

I suggest you reorder your migration file following ways:

Change the dates that form the first part of the migration filenames So they're in the order you want (example: for 2020_07_28_133303_update_categories.php, the date & time is 2020-07-28, 13:33:03);

N.B: First must be 'categories' migration file than 'meals' migration File.

Answer 4

if you are using ->onDelete('set null') in your foreign key definition make sure the foreign key field itself is nullable() ie

//Column definition
$table->integer('user_id')->unsigned()->index()->nullable(); //index() is optional

//...
//...

//Foreign key 
$table->foreign('user_id')
      ->references('id')
      ->on('users')
      ->onDelete('set null');

Answer 5

The order of creation of migration files should be sorted and the foreign key should have exactly similar property as the primary key in the other table.

Answer 6

Migrations must be created top-down.

First create the migrations for the tables who don't belong to anyone.

Then create the migrations for tables that belong to the previous.

---

Simplified answer to the table engine problem:

To set the storage engine for a table, set the engine property on the schema builder:

Schema::create('users', function ($table) {
    $table->engine = 'InnoDB';

    $table->increments('id');
});

From Laravel Docs: https://laravel.com/docs/5.2/migrations