Laravel Eloquent Relationships $with Property HTTP 500 Error

Laravel's Eager loading is a technique that helps eliminate the "N + 1" query problem in relationships. By default, relationships are loaded lazily in Laravel, which can result in significant loading time for large record sets. To overcome this, Laravel provides various methods for Eager loading relationships. In this blog post, we will focus on Eager Loading By Default, specifically using the $with property to specify relationships to be loaded automatically on the model.

Eager loading is a powerful feature in Laravel that significantly improves performance by reducing database queries. However, incorrect usage of the $with property can lead to unexpected errors, such as the HTTP 500 Error. To resolve this issue, we need to analyze potential causes and find a suitable solution.

Laravel Eager loading

Problem Statement : HTTP 500 Error when using Laravel Eloquent $with Property

While I have frequently used the $with property to define relationships to be loaded, thus reducing the code size by avoiding the need to add the with() method each time I call the eloquent model, I encountered an unexpected HTTP 500 Error when implementing this approach in one of my projects. The problematic code is provided below:

//Subcategory.php

namespace App\Models;

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

class SubCategory extends Model
{
    use HasFactory;


    //Relationships
    protected $with = ['courses'];

    public function courses()
    {
        return $this->hasMany(Course::class);
    }

}

//Course.php

namespace App\Models;

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

class Course extends Model
{
    use HasFactory;

    //Relationships
    protected $with = ['subCategory', 'instructor', 'category'];



    public function subCategory()
    {
        return $this->belongsTo(SubCategory::class);
    }

    public function instructor()
    {
        return $this->belongsTo(User::class, 'instructor_id');
    }

    public function category()
    {  //https://github.com/laravel/ideas/issues/1170#issuecomment-851409037
        return $this->hasOneThrough(Category::class, SubCategory::class, 'id', 'id', 'sub_category_id', 'category_id');
    }

}

In the above code, I attempted to use the $with property to eager load the 'subCategory', 'instructor', and 'category' relationships. When attempting to retrieve data using Course::all(), an unexpected HTTP 500 Error occurs. Surprisingly, using Course::with(['subCategory', 'instructor', 'category'])->get() successfully retrieves the data. It is important to note that both approaches specify the same Eloquent relationships. Through research, I discovered that having a $with property in two models that reference each other leads to a recursive loop in Eloquent, causing the HTTP 500 Error. In this blog post, I will share my findings and provide a solution to prevent this recursive loop.

Problem Explanation

In my case, when executing Course::all(), the Course model retrieves the data and attempts to load its relationships. The subCategory relationship triggers the SubCategory model, which, by default, attempts to load the courses relationship. This creates an endless loop that consumes PHP's memory, resulting in the HTTP 500 Error.

Solution to HTTP 500 Error when using Laravel Eloquent $with Property

To break the recursive loop, we need to modify the SubCategory.php file by removing the $with property that specifies the eager loading of the courses relationship. By doing this, the modified code would be as follows:

//Subcategory.php

namespace App\Models;

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

class SubCategory extends Model
{
    use HasFactory;


    //Relationships
   // Remove the $with property to prevent eager loading of the 'courses' relationship by default

    // ...

    public function courses()
    {
        return $this->hasMany(Course::class);
    }

}

By removing the $with property in the SubCategory model, we prevent the automatic eager loading of the courses relationship and effectively resolve the recursive loop issue.

Conclusion

In this blog post, we discussed the benefits of Eager loading relationships in Laravel and focused on Eager Loading By Default using the $with property. We also explored the scenario where an HTTP 500 Error occurs when using the Course::all() method due to a recursive loop caused by the $with property in Eloquent models. We identified the root cause of the error and presented a solution by modifying the SubCategory model to remove the $with property that loads the courses relationship by default. Implementing this solution prevents the endless loop and resolves the HTTP 500 Error. Ensure to apply this fix whenever you encounter a similar issue with recursive relationships in Laravel's Eloquent ORM.