Mastering Eloquent Relationships in Laravel

Understanding Relationships

Eloquent makes working with database relationships elegant and intuitive. Let's explore the different types of relationships available.

One-to-Many Relationships

The most common relationship type. A user has many posts, a post belongs to a user.

<?php

namespace App\Models;

use Illuminate\Database\Eloquent\Model;
use Illuminate\Database\Eloquent\Relations\HasMany;
use Illuminate\Database\Eloquent\Relations\BelongsTo;

class User extends Model
{
    public function posts(): HasMany
    {
        return $this->hasMany(Post::class);
    }
}

class Post extends Model
{
    public function author(): BelongsTo
    {
        return $this->belongsTo(User::class, 'user_id');
    }
}

Many-to-Many with Pivot Data

When you need additional data on the pivot table:

<?php

namespace App\Models;

use Illuminate\Database\Eloquent\Model;
use Illuminate\Database\Eloquent\Relations\BelongsToMany;

class User extends Model
{
    public function roles(): BelongsToMany
    {
        return $this->belongsToMany(Role::class)
            ->withPivot(['assigned_at', 'assigned_by'])
            ->withTimestamps()
            ->using(RoleUser::class);
    }

    public function assignRole(Role $role, User $assigner): void
    {
        $this->roles()->attach($role->id, [
            'assigned_at' => now(),
            'assigned_by' => $assigner->id,
        ]);
    }
}

Polymorphic Relationships

When multiple models can have the same relationship:

<?php

namespace App\Models;

use Illuminate\Database\Eloquent\Model;
use Illuminate\Database\Eloquent\Relations\MorphMany;
use Illuminate\Database\Eloquent\Relations\MorphTo;

class Comment extends Model
{
    public function commentable(): MorphTo
    {
        return $this->morphTo();
    }
}

class Post extends Model
{
    public function comments(): MorphMany
    {
        return $this->morphMany(Comment::class, 'commentable');
    }
}

class Video extends Model
{
    public function comments(): MorphMany
    {
        return $this->morphMany(Comment::class, 'commentable');
    }
}

Eager Loading

Avoid the N+1 query problem with eager loading:

<?php

// Bad - N+1 problem (101 queries for 100 posts)
$posts = Post::all();
foreach ($posts as $post) {
    echo $post->author->name;
}

// Good - Eager loading (2 queries)
$posts = Post::with('author')->get();
foreach ($posts as $post) {
    echo $post->author->name;
}

// Nested eager loading
$posts = Post::with([
    'author',
    'comments.user',
    'tags',
])->get();

// Constrained eager loading
$posts = Post::with([
    'comments' => function ($query) {
        $query->where('approved', true)
              ->latest()
              ->limit(5);
    },
])->get();

Conclusion

Understanding relationships deeply will make you a more effective Laravel developer. Always use eager loading to prevent N+1 queries, and leverage pivot tables for complex many-to-many relationships.