Eloquent Foreign Keys: 3 Common Mistakes to Avoid

Adding foreign keys can sometimes be tricky. You might get an error message or see that it doesn't work as expected.

There are 3 common mistakes that we see developers make when adding foreign keys to their databases:

• Forgetting to add Constraints
• Adding new Foreign Keys Without Proper Default Data
• Allowing to Delete Parent Records when Child Record Exists

Let's see those mistakes and how to fix them!

Mistake 1: Forgetting to add Constraints

Database constraints are a valuable feature we should not ignore. They help us protect from missing related records in our database.

Let's build a basic example here:

Migration

// Create Categories table
Schema::create('categories', function (Blueprint $table) {
    $table->id();
    $table->string('category')->nullable();
    $table->timestamps();
});
 
// Create a child Questions table with a relationship to the parent Categories table
Schema::create('questions', function (Blueprint $table) {
    $table->id();
    $table->foreignId('category_id');
    $table->longText('question')->nullable();
    $table->longText('answer')->nullable();
    $table->timestamps();
});

You might think that we are good to go here. However, we are not. It allows us to create new Questions with a category that doesn't exist:

$question = Question::create([
    'category_id' => 155,
    'question' => 'How to use Laravel?',
    'answer' => 'You can use Laravel by following the documentation',
]);

And this will work perfectly fine. Why? Because we don't tell our database to check for the category id.

We have 0 categories in our database, but we can still create a new Question with category id 155. It is not good. Let's fix it!

Migration

// ...
 
// Create a child Questions table with a relationship to the parent  Categories table
Schema::create('questions', function (Blueprint $table) {
    $table->id();
    $table->foreignId('category_id')->constrained();
    $table->longText('question')->nullable();
    $table->longText('answer')->nullable();
    $table->timestamps();
});

When we try to run the same code of Question::create(), we get an error...