Relationships in Laravel (hasOne & belongsTo)

Laravel provides various types of relationships to define the associations between different models.

1. One-to-One Relationship :
A one-to-one relationship is used when a single record in one model is associated with a single record in another model.

Example: User and Address
Each user has one address, and each address belongs to one user.
Define hasOne in the User Model:

public function address() {
    return $this->hasOne(Address::class, 'user_id', 'id'); // 'user_id' is the foreign key in the 'addresses' table
}

Fetch Users with Their Addresses:

Route::get('/get-users', function() {
    $users = User::with('address')->get();
    return view('user', ['users' => $users]);
});

Define belongsTo in the Address Model:

public function user() {
    return $this->belongsTo(User::class, 'user_id', 'id'); // 'user_id' links addresses to users
}

Fetch Address with Their Users:

Route::get('/get-addresses', function() {
    $addresses = Address::with('user')->get();
    return view('address', ['addresses' => $addresses]);
});

2. One-to-Many (hasMany()) :
A one-to-many relationship is used when one record in a model is associated with multiple records in another model. For example, a user can have multiple posts, but each post belongs to one user.

Example: User and Posts
1. In the app\models\User model:

public function posts() {
    return $this->hasMany(Post::class, 'user_id', 'id'); // 'user_id' is the foreign key in the 'posts' table
}

1. In the app\models\Post model:

public function user() {
    return $this->belongsTo(User::class, 'user_id', 'id'); // 'user_id' links posts to users
}

Fetch Users with Their Posts and Addresses:

Route::get('/get-users', function() {
    $users = User::has('posts') // Filters users who have at least one post
                 ->with(['address', 'posts']) // Eager loads related data
                 ->get();
    return view('user', ['users' => $users]);
});

Fetch Posts with Their Associated Users:

Route::get('/get-posts', function() {
    $posts = Post::with('user') // Eager loads the user associated with each post
                 ->limit(20) // Limits the result to 20 posts
                 ->get();
    return view('posts', ['posts' => $posts]);
});

Testing in Blade Views user.blade.php :

@foreach($users as $user)
    <h2>{{ $user->name }}</h2>
    <p>Address: {{ $user->address->street ?? 'No address' }}</p>
    <h3>Posts:</h3>
    <ul>
        @foreach($user->posts as $post)
            <li>{{ $post->title }} - {{ $post->created_at }}</li>
        @endforeach
    </ul>
@endforeach

3. Many-to-Many (belongsToMany()) :
A many-to-many relationship is used when records in one model can be associated with multiple records in another model, and vice versa. For example:

• A Post can have multiple Tags.
• A Tag can have multiple Posts.
• dd

A pivot table is used to manage this relationship. This table acts as an intermediary, containing the foreign keys of both models.

Step-by-Step Implementation

1. Defining Relationships in Models

•  In the app\models\Post model:

public function tags() {
    return $this->belongsToMany(Tag::class, 'post_tag', 'post_id', 'tag_id');
}

• post_tag: The pivot table name.
• post_id: Foreign key for the posts table.
• tag_id: Foreign key for the tags table.
   

• In the Tag Model:

public function posts() {
    return $this->belongsToMany(Post::class, 'post_tag', 'tag_id', 'post_id');
}

2. Fetching Data

• Get Posts with Tags:

Route::get('/posts', function() {
    $posts = Post::with('tags')->get();
    return view('post.index', ['posts' => $posts]);
});

• Get Tags with Posts:

Route::get('/tags', function() {
    $tags = Tag::with('posts')->get();
    return view('tag.index', ['tags' => $tags]);
});

3. Blade Views

•  Display Posts and Their Tags: post/index.blade.php

@foreach ($posts as $post)
    <h2>{{ $post->title }}</h2>
    <ul>
        @foreach ($post->tags as $tag)
            <li>{{ $tag->name }}</li>
        @endforeach
    </ul>
@endforeach

• Display Tags and Their Posts: tag/index.blade.php

@foreach ($tags as $tag)
    <h2>{{ $tag->name }}</h2>
    <ul>
        @foreach ($tag->posts as $post)
            <li>{{ $post->title }}</li>
        @endforeach
    </ul>
@endforeach

4. Creating the Pivot Table

Run the following Artisan command to create the post_tag pivot table migration:

php artisan make:migration create_post_tag_table --create=post_tag