Foreign key is incorrectly formated – Laravel 7 migration

This is my first Laravel project and I’m having a hard time defining a table with a many-to-many relationship.

I want to create a table which will ‘link’ each teacher from a table with one or more domains they are interested in. Each domain can be linked to multiple teachers.

I approached the idea this way:

<?php

use IlluminateDatabaseMigrationsMigration;
use IlluminateDatabaseSchemaBlueprint;
use IlluminateSupportFacadesSchema;

class teacherAndTheirDomain extends Migration
{
    /**
     * Run the migrations.
     *
     * @return void
     */
    public function up()
    {
        Schema::create('teacherAndTheirDomain', function (Blueprint $table) {
            $table->unsignedInteger('teacher_id');
            $table->unsignedInteger('domain_id');
            
            $table->foreign('teacher_id')
                  ->references('id')->on('teachers')
                  ->onDelete('cascade');
            $table->foreign('domain_id')
                  ->references('id')->on('domains')
                  ->onDelete('cascade');
        });
    }

    /**
     * Reverse the migrations.
     *
     * @return void
     */
    public function down()
    {
        Schema::dropIfExists('teacherAndTheirDomain');
    }
}

but I get “Can’t create table, foreign key constraint is incorrectly formed”.

Can anybody explain to me what I’m doing wrong, please, and why am I getting this error?

Thanks 🙂

Answers:

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Method 1

Foreign key constraints must be of the same type as the referencing table. As explained on ‘Conditions and Restrictions’ on https://dev.mysql.com/doc/refman/5.6/en/create-table-foreign-keys.html

Since Laravel 7, the default is set to $table->bigIncrements('id'); instead of $table->increments('id');. Hence, the foreign key should be of the same type:

$table->unsignedBigInteger('teacher_id');
$table->unsignedBigInteger('domain_id');

The rest of the migration file stays the same.

ps: you don’t need to use big integers as the table id type. In most circumstances, falling back on the old $table->increments('id'); will be just fine. Once you made a choice, try to be consistent in all other migrations to avoid future conflicts in your application.

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