Defining and Using Laravel Eloquent Relationships (One-to-One, Inverse, and Default Models)

Database tables are often related (e.g., a blog post has many comments, an order belongs to a user). Laravel's Eloquent ORM makes managing these relationships simple and supports various types such as one-to-one, one-to-many, many-to-many, polymorphic, and more.

Relationships are defined as methods on Eloquent model classes. Like the models themselves, relationships can be used as powerful query builders, allowing chainable constraints. For example, you can add a condition to the posts relationship:

$user->posts()->where('active', 1)->get();

One-to-One Relationship

A one-to-one relationship links a single record in one table to a single record in another. For instance, a User model may have one associated Phone model. Define the relationship in the User model:

<?php

namespace App;

use Illuminate\Database\Eloquent\Model;

class User extends Model
{
    /**
     * Get the phone record associated with the user.
     */
    public function phone()
    {
        return $this->hasOne('App\Phone');
    }
}

After defining the method, you can access the related phone via a dynamic property:

$phone = User::find(1)->phone;

Eloquent determines the foreign key name based on the model name, assuming Phone has a user_id column. To override the default foreign key, pass a second argument to hasOne :

return $this->hasOne('App\Phone', 'foreign_key');

To also customize the local key, provide a third argument:

return $this->hasOne('App\Phone', 'foreign_key', 'local_key');

Defining the Inverse Relationship

Now define the inverse on the Phone model so you can retrieve the owning User :

<?php

namespace App;

use Illuminate\Database\Eloquent\Model;

class Phone extends Model
{
    /**
     * Get the user that owns this phone.
     */
    public function user()
    {
        return $this->belongsTo('App\User');
    }
}

If the foreign key on Phone is not user_id , specify it as the second argument:

return $this->belongsTo('App\User', 'foreign_key');

When the parent model uses a custom primary key or you need a different column for the relationship, provide a third argument:

return $this->belongsTo('App\User', 'foreign_key', 'other_key');

Default Models with withDefault

The belongsTo relationship can define a default model that is returned when the related record does not exist, implementing the Null Object pattern:

public function user()
{
    return $this->belongsTo('App\User')->withDefault();
}

You can also supply an attribute array to populate the default model:

return $this->belongsTo('App\User')->withDefault([
    'name' => '游客',
]);

Or use a closure for more complex default initialization:

return $this->belongsTo('App\User')->withDefault(function ($user) {
    $user->name = '游客';
});

These techniques allow you to work with relationships safely without needing additional null checks.