How to add Foreign key in Migration - Laravel

In Laravel, migrations are a way to modify and manage the database schema of the application. A foreign key is a field that is used to establish the relationship between two tables via the primary key (You can also use a non-primary field but not recommended).

In this tutorial, I show how you can add a foreign key constraint while creating a table using migration in the Laravel 9 project.

Database Configuration
Create Table and add Foreign Key

Model
Conclusion

1. Database Configuration

Open .env file.

Specify the host, database name, username, and password.

DB_CONNECTION=mysql
DB_HOST=127.0.0.1
DB_PORT=3306
DB_DATABASE=tutorial
DB_USERNAME=root
DB_PASSWORD=

2. Create Table and add Foreign Key

Create countries, states, and cities tables using migration.

I am adding foreign key on states and cities tables.

states table is linked to countries table, and
cities table is linked to states table.

Create Countries table –

php artisan make:migration create_countries_table

Now, navigate to database/migrations/ folder from the project root.
Find a PHP file that ends with create_countries_table and open it.
Define the table structure in the up() method.

public function up()
{
    Schema::create('countries', function (Blueprint $table) {
        $table->id();
        $table->string('name');
        $table->timestamps();
    });
}

Create States table –

php artisan make:migration create_states_table

Similarly, find a PHP file that ends with create_states_table in database/migrations/ folder and open it.
Define the table structure in the up() method.

Adding foreign key –

I am adding a foreign key to country_id field.
Set datatype of this field to UNSIGNED BIGINT – $table->unsignedBigInteger('country_id');. Datatype must be UNSIGNED and same as the parent table linking field datatype.

Here, countries table id field has biginteger datatype.
Add foreign key –

$table->foreign('country_id')
     ->references('id')->on('countries')->onDelete('cascade');

Values –

foreign() – Pass field name which you want to foreign key constraint.
references() – Pass linking table field name.
on() – Linking table name.

onDelete(‘cascade’) – Enable deletion of attached data.

public function up()
{
    Schema::create('states', function (Blueprint $table) {
         $table->id();
         $table->unsignedBigInteger('country_id');
         $table->string('name');
         $table->timestamps();
         $table->foreign('country_id')
              ->references('id')->on('countries')->onDelete('cascade');
    });
}

Create Cities table –

php artisan make:migration create_cities_table

Similarly, find a PHP file that ends with create_cities_table in database/migrations/ folder and open it.

Define the table structure in the up() method.

Adding foreign key –

I am adding a foreign key to states_id field.

Set datatype of this field to UNSIGNED BIGINT – $table->unsignedBigInteger('state_id');. Datatype must be UNSIGNED and same as the parent table linking field datatype.
Here, states table id field has biginteger datatype.
Add foreign key –

$table->foreign('state_id')
     ->references('id')->on('states')->onDelete('cascade');

Values –

foreign() – Pass field name which you want to foreign key constraint.
references() – Pass linking table field name.

on() – Linking table name.
onDelete(‘cascade’) – Enable deletion of attached data.

public function up()
{
    Schema::create('cities', function (Blueprint $table) {
         $table->id(); 
         $table->unsignedBigInteger('state_id');
         $table->string('name');
         $table->foreign('state_id')
                  ->references('id')->on('states')->onDelete('cascade');
         $table->timestamps();
    });
}

Run the migration to create tables –

php artisan migrate

3. Model

Create Countries, States, and Cities models.

Create Countries Model.

php artisan make:model Countries

Open app/Models/Countries.php file.

Specify mass assignable Model attributes – name using the $fillable property.

Completed Code

<?php

namespace App\Models;

use Illuminate\Database\Eloquent\Factories\HasFactory;
use Illuminate\Database\Eloquent\Model;

class Countries extends Model
{
      use HasFactory;

      protected $fillable = [
          'name'
      ];
}

Create States Model.

php artisan make:model States

Open app/Models/States.php file.
Specify mass assignable Model attributes – country_id, and name using the $fillable property.

Completed Code

<?php

namespace App\Models;

use Illuminate\Database\Eloquent\Factories\HasFactory;
use Illuminate\Database\Eloquent\Model;

class States extends Model
{
      use HasFactory;

      protected $fillable = [
           'country_id','name'
      ];
}

Create Cities Model.

php artisan make:modelCities

Open app/Models/Cities.php file.
Specify mass assignable Model attributes – state_id, and name using the $fillable property.

Completed Code

<?php

namespace App\Models;

use Illuminate\Database\Eloquent\Factories\HasFactory;
use Illuminate\Database\Eloquent\Model;

class Cities extends Model
{
      use HasFactory;

      protected $fillable = [
         'state_id','name'
      ];
}

4. Conclusion

Adding a foreign key to the table using migration in Laravel is a straightforward process. By following these steps, you can easily define a foreign key constraint and maintain referential integrity between tables in your database.

In the example, I added a single foreign key to a table but you can add more than one foreign key to a table by following the same steps.

Follow the same steps if you are on Laravel 8.

View this official document to know more about foreign key implementation.

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How to add Foreign key in Migration – Laravel

Contents

1. Database Configuration

2. Create Table and add Foreign Key

3. Model

4. Conclusion