Inverse BelongsToThrough relationships in Laravel models

No, Laravel doesn’t actually have a BelongsToThrough relationship, a common frustration for models where you want to access the top level of a BelongsTo chain.

Consider this setup:

1class User extends Model
2{
3	public function posts(): HasMany
4	{
5		return $this->hasMany(Post::class);
6	}
7}
8
9class Post extends Model
10{
11	public function reviews(): HasMany
12	{
13		return $this->hasMany(Review::class);
14	}
15}
16
17class Review extends Model
18{
19	public function postAuthor()
20	{
21		// Now what?
22	}
23}

A user has authored many posts. Those posts have many reviews. So how do we define the inverse relationship back up from Review -> User (the post author) without having to load Post in the middle?

The Solution

Enter the hasOneThrough relationship, albeit slightly repurposed.

1class Review extends Model
2{
3	public function postAuthor(): HasOneThrough
4	{
5		return $this->hasOneThrough(User::class, Post::class, 'id', 'id', 'post_id', 'user_id');
6	}
7}

The Explanation

The hasOneThrough relationship isn’t designed for inverse relationships as you’ll see when comparing our solution with the documentation:

// Reproduced from https://laravel.com/docs/10.x/eloquent-relationships#has-one-through
class Mechanic extends Model
{
    /**
     * Get the car's owner.
     */
    public function carOwner(): HasOneThrough
    {
        return $this->hasOneThrough(
            Owner::class,
            Car::class,
            'mechanic_id', // Foreign key on the cars table...
            'car_id', // Foreign key on the owners table...
            'id', // Local key on the mechanics table...
            'id' // Local key on the cars table...
        );
    }
}

Because we want to invert the relationship, we have to invert the foreign keys for local keys and the local keys for foreign keys. So ours works out at:

class Review extends Model
{
	public function postAuthor(): HasOneThrough
	{
		return $this->hasOneThrough(
			User::class, // target class
			Post::class, // through class
			'id', // local key on the posts table
			'id', // local key on the users table
			'post_id', // foreign key on the reviews table
			'user_id' // foreign key on the posts table
		);
	}
}