Querying from Many-to-Many Relationship in Laravel

Laravel is a popular PHP framework known for its simplicity, flexibility, and ease of use. One common issue developers face when working with many-to-many relationships is querying the data efficiently. In this article, we’ll explore how to query from many-to-many relationship tables using Laravel’s Eloquent ORM.

Introduction to Many-to-Many Relationships

In a many-to-many relationship, two models (in our case, Classes and Subjects) have a third model (often referred to as the pivot table) that acts as an intermediary between them. The pivot table allows for multiple instances of one model to be related to another.

For example, consider a class has multiple subjects assigned to it, and each subject can have multiple classes associated with it. The resulting database schema would look something like this:

• classes table:

  • id
  • name

• subjects table:

  • id
  • name

• class_subjects pivot table (many-to-many relationship):

  • class_id
  • subject_id
  • created_at
  • updated_at

This many-to-many relationship can be achieved in Laravel by defining the relationship methods on both models.

Defining the Relationship Methods

In your model files, you should define a method to establish the relationship between two models. Here’s how it’s done for our example:

// app/Models/Class.php
namespace App\Models;

use Illuminate\Database\Eloquent\Model;

class Class extends Model
{
    public function subjects()
    {
        return $this->belongsToMany(Subject::class);
    }
}

// app/Models/Subject.php
namespace App\Models;

use Illuminate\Database\Eloquent\Model;

class Subject extends Model
{
    public function classes()
    {
        return $this->belongsToMany(Class::class);
    }
}

Querying the Data

When querying from a many-to-many relationship, you need to join the two tables using their pivot table. Here’s an example of how it can be done:

// app/Http/Controllers/ClassController.php
namespace App\Http\Controllers;

use Illuminate\Http\Request;
use App\Models\Class;
use App\Models\Subject;

class ClassController extends Controller
{
    public function index()
    {
        $classes = Class::with('subjects')->get();
        
        return view('classes.index', ['classes' => $classes]);
    }
}

In the above code, Class::with('subjects') will perform an Eloquent query that includes the related subjects.

However, if you want to query for a specific subject’s classes, or vice versa, you can use eager loading with the load() method:

// app/Http/Controllers/ClassController.php
namespace App\Http\Controllers;

use Illuminate\Http\Request;
use App\Models\Class;
use App\Models\Subject;

class ClassController extends Controller
{
    public function index()
    {
        $classes = Class::with('subjects')->get();
        
        return view('classes.index', ['classes' => $classes]);
    }
    
    // Querying for a subject's classes
    public function showClassSubjects($id)
    {
        $class = Class::find($id);
        $subjectClasses = $class->subjects;
        
        return view('show-class-subjects', ['subjectClasses' => $subjectClasses]);
    }
}

And in your blade file:

<!-- resources/views/show-class-subjects.blade.php -->
<table>
    <thead>
    <tr>
        <th>Name</th>
        <th>Codes</th>
    </tr>
    </thead>
    <tbody>
    @foreach($subjectClasses as $subjectClass)
        <tr>
            <td>{{ $subjectClass->subject->name }} AS</td>
            <td>{{ implode(', ', $subjectClass->class->subjects) }}</td>
        </tr>
    @endforeach
    </tbody>
</table>

Dynamic Code Generation

When dealing with many-to-many relationships, it can become quite complicated to write dynamic code that correctly handles the queries. To avoid repetitive and lengthy code for different combinations of subjects and classes, you might need to look into using a library or framework like Laravel’s laracasts/laravel-discoverable-models package.

However, if this isn’t feasible, there is an alternative approach:

// resources/views/show-class-subjects.blade.php
<table>
    <thead>
    <tr>
        <th>Name</th>
        <th>Codes</th>
    </tr>
    </thead>
    <tbody>
    @foreach($classes as $class)
        <tr>
            <td>{{ $class->name }}</td>
            <td>{{ implode(', ', array_unique(array_merge($class->subjects->pluck('name')->toArray(), $class->subjects->withTimestamps()->pluck('name')->toArray()))) }}</td>
        </tr>
    @endforeach
    </tbody>
</table>

This will provide a dynamic and correct solution to the problem, but be aware that it involves more complex logic.

Conclusion

Querying from many-to-many relationship tables can seem daunting at first, but once you grasp the basics of Eloquent’s with() method and how to use eager loading, querying data becomes much simpler.

Last modified on 2023-09-26