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Table of Contents
1. Understand common Eloquent relationship types
2. How to define a "one-to-many" relationship?
3. Pay attention to the intermediate table when defining a "many-to-many" relationship
4. Use with() to preload to avoid N 1 query problems
Home PHP Framework Laravel How to Define Eloquent Relationships in Laravel?

How to Define Eloquent Relationships in Laravel?

Jul 12, 2025 am 01:28 AM

The key to defining model relationships using Eloquent ORM in Laravel is to understand common relationship types and set them correctly. 1. Common relationships include one-to-one, one-to-many, belongsToMany, far-level one-to-many (hasMany) and polymorphic relationships; 2. One-to-many relationships are defined by the hasMany method, and the primary key id is matched by the default to the foreign key user_id, and foreign keys can also be specified manually; 3. Many-to-many relationships require intermediate tables and are defined by belongsToMany, which can specify intermediate table names and load additional fields withPivot; 4. Use with() to preload to avoid N 1 query problems, and support nested loading and conditional loading.

How to Define Eloquent Relationships in Laravel?

In Laravel, Eloquent ORM provides a very convenient way to handle relationships between database models. Once these relationships are defined, you can easily access the associated data without writing complex query statements. The key is to understand several common relationship types and set them correctly in the model.

How to Define Eloquent Relationships in Laravel?

1. Understand common Eloquent relationship types

Laravel supports relationships between multiple models, and the most commonly used are the following:

How to Define Eloquent Relationships in Laravel?
  • One to One : For example, a user has a mobile phone number.
  • One To Many : For example, a user can have multiple orders.
  • Many to many : For example, there is a many-to-many relationship between a user and a role. A user can have multiple roles, and a role can also be assigned to multiple users.
  • Has Many Through : For example, the user can obtain the comment content of all his orders.
  • Polymorphic Relations : used in situations where a model can belong to multiple other models, such as comments can belong to articles or videos.

Each relationship has the corresponding Eloquent method to define, such as hasOne , hasMany , belongsToMany , etc.


2. How to define a "one-to-many" relationship?

This is one of the most common relationships. Suppose you have two models: User and Post, and one user can publish multiple articles.

How to Define Eloquent Relationships in Laravel?

Add the following method to the User model:

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

By default, Laravel automatically uses the user's primary key id to match the user_id field in the post table. If your data table fields are different, you can specify it manually:

 return $this->hasMany(Post::class, 'user_id_foreign');

Then you can call it like this:

 $user = User::find(1);
foreach ($user->posts as $post) {
    echo $post->title;
}

3. Pay attention to the intermediate table when defining a "many-to-many" relationship

Taking users and roles as examples, you need three tables: users, roles, and role_user (intermediate table).

Defined in the User model:

 public function roles()
{
    return $this->belongsToMany(Role::class);
}

Similarly, the default intermediate table is spliced ??alphabetically (role_user). If not, you can pass in parameters to specify:

 return $this->belongsToMany(Role::class, 'user_role');

If your intermediate table has additional fields, such as created_at or other information, you can load these fields through withPivot :

 return $this->belongsToMany(Role::class)->withPivot('created_at');

4. Use with() to preload to avoid N 1 query problems

When you loop through a model and access its relationship, if no preloading is done, each loop initiates a new query. This can lead to performance degradation.

The correct way to do this is to use with() when querying:

 $users = User::with('posts')->get();

foreach ($users as $user) {
    foreach ($user->posts as $post) {
        // No additional queries are generated here}
}

You can also nest loading:

 User::with('posts.comments')->get();

Or loading with conditions:

 User::with(['posts' => function($query) {
    $query->where('published', true);
}])->get();

Basically that's it. The key to defining an Eloquent relationship is to understand the logical structure between models and then select appropriate methods and parameters to express it. Although it may be a bit confusing at the beginning, you will find that it greatly simplifies database operations after getting familiar with it.

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