Eloquent Performance: Fake relationships

In the previous post I discussed the subqueries, an efficient way to fetch a subset of data from another table.

In this post, I will extend upon the subquery and introduce you to the fake queries.

Fake query is the name that I use to refer to a particular solution, other people may use different names.

Let me show you what I mean.

In the previous post, we've seen how to fetch the latest_login_date from the App\Models\Login in a fast way.

What if you want to get the latest App\Models\Login as an Eloquent model instance and not just a Carbon instance?

But isn't that easy to achieve? I mean we can add a method to fetch the last login in the App\Models\User as follows:

// App\Models\User

public function latestLogin()
{
    return App\Models\Login::where('user_id', $this->id)->latest()->first();
}

Bummm! we introduced N+1 problem 😄, since the App\Models\Login will be loaded for each user individually.

Let me show you how to solve such an issue by using the fake relationship.

Fake relationships

We can create a fake relationship and delude Laravel to treat it as if it was a real relationship, sounds confusing? No problem, let me demystify it.

Let's add a new fake relationship in the App\Models\User:

public function lastLogin()
{
    return $this->belongsTo(App\Models\Login::class);
}

As I said, this is a fake relationship, because there's no last_login_id in the App\Models\User which is required for the belongsTo relationship to work properly.

The solution is to combine the subquery with the eager-loading to make the lastLogin relationship works:

public function scopeWithLastLogin($query)
{
    $query->addSelect(['last_login_id' => Login::select('id')
        ->whereColumn('user_id', 'users.id')
        ->latest()
        ->take(1)
    ])->with('lastLogin');;
}

// App\Http\Controllers\UsersController

// ...

User::withLastLogin()->paginate();

// ...

If you take a look at the debugger you'll see that all the latest logins were eager-loaded:

Remember that the lastLogin method cannot be used without calling the withLastLogin scope on the App\Models\User.

In the next post, I will discuss the reusable relationships.