How to add Foreign key in Migration – Laravel 8

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 creation of table using migration in the Laravel 8 project.

How to add Foreign key in migration - Laravel 8


Contents

  1. Database Configuration
  2. Create Table and add Foreign Key
  3. Model
  4. 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/migration/ 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
  • Similary, find a PHP file that ends with create_states_table in database/migration/ folder and open it.
  • Define the table structure in the up() method.

Adding foreign key –

  • I am adding 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 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
  • Similary, find a PHP file that ends with create_cities_table in database/migration/ folder and open it.
  • Define the table structure in the up() method.

Adding foreign key –

  • I am adding 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 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 $filliable 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 $filliable 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 $filliable 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

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

You can learn more about it from here.

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