计算laravel中关系的关系

Question

Suppose I have a Conversation model like this :

class Conversation extends Model
{
    public function questions (){
        return $this->hasMany('App\Question','conversation_id','conversation_id');
    }
    public function category ()
    {
        return $this->belongsTo('App\Category', 'cat', 'cat_id');
    }

}

And a Question model like this:

class Question extends Model
{
    public function conversation ()
    {
        return $this->belongsTo('App\Conversation', 'conversation_id', 'conversation_id');
    }
}

As you can see there is a hasMany relation between those two.

In the other hand there is a Category like below that has a relation with Conversation model :

class Category extends Node
{
    public function conversations (){
        return $this->hasMany('App\Conversation','cat','cat_id');
    }
}

Now I want to append an attribute named question_count to Category that counts all questions of conversations of each category. for that I added this :

    public function getQuestionsCountAttribute ()
    {
        return $this->conversations->questions->count();
    }

But when fetch a category I got this error :

ErrorException in Category.php line 59:
Undefined property: Illuminate\Database\Eloquent\Collection::$questions

What did I do? how can I count relations of a relation with minimum server overloading?

I am using laravel 5.3.4.

Answer 1

I think that you need a has many through relationship here.

What you do wrong:

When you write $this->conversations->questions, this can't work, because the questions are a relation of a single conversation and not of a collection of conversations (here, $this->conversations is a Collection)

The solution:

Using hasManyThrough relation:

You can find the documentation for this relation on this page, if my explanation is bad

The basics are, you need to define a relation on your Category model:

class Category extends Node
{
    public function conversations ()
    {
        return $this->hasMany('App\Conversation');
    }

    public function questions ()
    {
        return $this->hasManyThrough('App\Question', 'App\Conversation');
    }
}

(I will let your look into the documentation for your non standards foreign keys)

You should then be able to use: $category->questions->count()