Setting a foreign key bigInteger to bigIncrements in Laravel 5.4
Stefan Bogdanescu
Founder & Senior Architect · 2026-06-29
# Setting a Foreign Key: Resolving Type Mismatches in Laravel Migrations
As developers moving between different database systems or evolving schema designs, managing data types and foreign key constraints can often lead to subtle yet frustrating errors, like the "foreign key is incorrectly formed" error you encountered. This issue usually stems from a mismatch between how the parent table defines its primary key and how the child table attempts to reference it.
This post will walk through the specific problem presented in your migration example and provide a robust, developer-focused solution, ensuring schema integrity when using `bigIncrements` with foreign keys in Laravel.
## Understanding the Root of the Error
You are attempting to link the `users` table (which uses `bigIncrements('id')`) to the `social_logins` table using a column defined as `bigInteger('user_id')`. While both types deal with large integers, the underlying database constraint requires that the data type and signedness of the referencing column exactly match the primary key it references.
In MySQL (which Laravel commonly uses), `bigIncrements()` automatically creates an `BIGINT UNSIGNED` primary key. When you define a foreign key reference, the referencing column *must* also be an unsigned integer type to maintain referential integrity across the tables. If the types don't align perfectlyâfor instance, if one is signed and the other is unsigned, or if the precision differsâthe database engine rejects the constraint creation.
## The Correct Approach: Aligning Data Types in Migrations
The key to avoiding this error lies in adhering to best practices for defining primary keys and foreign keys within your Laravel migrations. Instead of manually specifying generic types like `bigInteger()`, it is highly recommended to use Laravel's expressive methods, which handle the necessary unsignedness automatically.
### 1. Correcting the Parent Table (`users`)
Your definition for the `users` table looks correct:
```php
Schema::create('users', function (Blueprint $table) {
$table->engine = 'InnoDB';
$table->bigIncrements('id')->unsigned(); // This creates BIGINT UNSIGNED
// ... other fields
});
```
### 2. Correcting the Child Table (`social_logins`)
To successfully create a foreign key relationship, the `user_id` column in `social_logins` must be defined as an unsigned integer that perfectly matches the parent's `id`. The most idiomatic way to achieve this in Laravel is by using `unsignedBigInteger()`. This method ensures the column is correctly typed for large IDs (up to 18 quintillion) and aligns with the `bigIncrements()` definition from the parent table.
Here is the corrected migration for the `social_logins` table:
```php
public function up()
{
Schema::create('social_logins', function (Blueprint $table) {
$table->engine = 'InnoDB';
$table->bigIncrements('id');
// CORRECTED LINE: Use unsignedBigInteger() to match the parent's BIGINT UNSIGNED ID
$table->unsignedBigInteger('user_id');
$table->foreign('user_id')->references('id')->on('users')->onDelete('cascade');
$table->string('provider', 32);
$table->string('provider_id');
$table->string('token')->nullable();
$table->string('avatar')->nullable();
$table->timestamps();
});
}
```
By using `$table->unsignedBigInteger('user_id')`, you explicitly tell the database that `user_id` will store positive, unsigned 64-bit integers, which perfectly aligns with the `bigIncrements('id')` column in the `users` table. This strict alignment is what satisfies the foreign key constraint validation and prevents the "incorrectly formed" error.
## Best Practices for Relational Design
When designing relational databases using Laravel, remember that Eloquent models rely heavily on these underlying schema definitions. Always prioritize defining primary keys as auto-incrementing unsigned big integers when dealing with large datasets. This consistency is fundamental to writing maintainable and scalable applications. For more advanced guidance on database interactions within the Laravel framework, explore resources from [laravelcompany.com](https://laravelcompany.com).
## Conclusion
The error you faced was a classic case of schema type misalignment during foreign key creation. The solution is simple yet critical: ensure that the column used for the foreign key reference (`user_id`) has the exact same data type and signedness as the primary key it points to (`users.id`). By switching from generic `bigInteger()` to Laravel's specific `unsignedBigInteger()`, you ensure database integrity, prevent runtime errors, and follow modern migration best practices.