Foreign key validation rule

Blog post
by Tim MacDonald on the

In this post I'm going to show you how you can validate foreign keys in your request form objects by calling:


I'm digging it. It's just a simple extension on the exists rule, but I think it is killer.

Find or fail?

When I started with Laravel, I always found it quick and easy to utilise the Eloquent query builder's findOrFail method to ensure that any foreign keys I was submitted from a user were valid. Most of the time these foreign keys are coming from a HTML select drop-down in some web form. Of course we want to fail to make sure we don't attempt to insert, into the database, a foreign key that doesn't exist (from some tricksy little hobbits'zs that decide they want to wreak havoc in our apps), but was this the best approach?

Then I got thinking, really the findOrFail method is just for resources in the URL. That being said - I really don't care what type of error I send someone being malicious.

Say we have a user in the database with an id of 2. That user has a foreign key of role_id.

If this user is being updated by calling PATCH: /users/2, but the request contains a role_id value that does not exists, it doesn't really make sense that the response would be a 404 error because that user does exist. They should, of course, receive a validation error. But findOrFail did serve the general purpose for me in the early days.

Validation forms

Keeping all your validation in a form object makes things much easier to maintain. If I'm using a form object and still utilising findOrFail in the controller for my role_id foreign key, obviously I haven't validated my data. I should trust that those keys exist, and can just lean on the find method instead.

Exists rule

Luckily, Laravel offers a handy validation rule exists to make this easy. As an initial introduction, say we are creating a password reset controller. The request coming in will contain an email address, and we need to check that the email address exists in the database for some user. We might do something like:

$rules = [
    'email' => 'required|email|exists:users,email',

This will hit the users table and search through the email column for the value in the request for email. If it does not find that email I can present the user with an error letting them know the email address does not exist.

Exists with foreign key, the stringy way

Translating this to a foreign key rule is pretty straightforward. We just need to specify the table and column.

My current project has a User and Subscription class. The user has a subscription_id foreign key field. Let's see how we could validate a submitted subscription_id value using the exists rule:

$rules = [
    'subscription_id' => 'exists:subscriptions,id',

Here we are saying, ensure that the request input subscription_id exists in the subscriptions table by looking for it in the id column.

Perfect! Exactly what we were after. But wait - there's more!

Wouldn't it be annoying if we change our table name or primary key column name later on? Then we would have to update these strings manually. But we could always just access those values from an Eloquent instance instead:

$subscription = new Subscription;

$rules = [
    'subscription_id' => 'exists:'.$subscription->getTable().','.$subscription->getKeyName(),

Hmmm. Although it works, I can't handle this. It's just to messy for me.

Exists with foreign key, the fluent way

Let's utilise Laravel's built in fluent validation rules. These are nice helpers around some of the more complex validation rules to make it easier to do more complex computation within the rules.

use Illuminate\Validation\Rule;

$subscription = new App\Subscription;

$rules = [
    'subscription_id' => [
        Rule::exists($subscription->getTable(), $subscription->getKeyName()),

Okay, now we are getting somewhere, but I think we can do better!

Extending the validator

Let's solidify our concept of foreign key validation to make this really clear by extending the Laravel validator, you should do this in a service provider.

We're going to allow you to pass in an Eloquent instance or a class string. It's going to be awesome...


namespace App\Providers;

use Illuminate\Validation\Rules\Exists;
use Illuminate\Support\ServiceProvider;
use Illuminate\Support\Facades\Validator;

class AppServiceProvider extends ServiceProvider
    public function boot()
        Validator::extend('foreign_key', function ($attribute, $value, $parameters, $validator) {
            $instance = is_string($parameters[0]) ? new $parameters[0] : $parameters[0];

            return new Exists($instance->getTable(), $instance->getKeyName());

Using the foreign key validation rule

We can now access our new rule in our form validation. To kick things off, let's do it the stringy way.

$rules = [
    'subscription_id' => 'foreign_key:'.App\Subscription::class,

So that will work fine, but again - we can do better! There are a couple of ways we can do this. The first is to extend the Illuminate\Validation\Rule class and add the method there, or add a macro to it, as it utilises the Macroable trait, but I'm going to use this as a chance to flaunt the Laravel Validation Rule package I made.

Once you have that installed, just extend the package like so:


You should do this just after you extend the validator in your service provider.

Now we have added all the magic sauce, lets see it in action. When we want to validate that a foreign key exists, we can simply use:

use App\Subscription;
use TiMacDonald\Validation\Rule;

$rules = [
    'subscription_id' => Rule::foreignKey(Subscription::class)->get(),

or assuming we already had an instance in the $subscription variable:

use TiMacDonald\Validation\Rule;

$rules = [
    'subscription_id' => Rule::foreignKey($subscription)->get(),


Laravel 5.5 added custom validation rules, which means the framework now gives you a way to do this really cleanly.