How to set Eloquent relationship belongsTo THROUGH another model in Laravel?

Question

I have a model Listing that inherits through its belongsTo('Model') relationship should inherently belong to the Manufacturer that its corresponding Model belongs to.

Here's from my Listing model:

    public function model()
    {
        return $this->belongsTo('Model', 'model_id');
    }

    public function manufacturer()
    {
        return $this->belongsTo('Manufacturer', 'models.manufacturer_id');
        /*
        $manufacturer_id = $this->model->manufacturer_id;
        return Manufacturer::find($manufacturer_id)->name;*/
    }

and my Manufacturer model:

public function listings()
{
    return $this->hasManyThrough('Listing', 'Model', 'manufacturer_id', 'model_id');
}

public function models()
{
    return $this->hasMany('Model', 'manufacturer_id');
}

I am able to echo $listing->model->name in a view, but not $listing->manufacturer->name. That throws an error. I tried the commented out 2 lines in the Listing model just to get the effect so then I could echo $listing->manufacturer() and that would work, but that doesn't properly establish their relationship. How do I do this? Thanks.

Revised Listing model (thanks to answerer):

    public function model()
    {
        return $this->belongsTo('Model', 'model_id');
    }

    public function manufacturer()
    {
        return $this->belongsTo('Model', 'model_id')
            ->join('manufacturers', 'manufacturers.id', '=', 'models.manufacturer_id');
    }

Answer 1

I found a solution, but it's not extremely straight forward. I've posted it below, but I posted what I think is the better solution first.

You shouldn't be able to access manufacturer directly from the listing, since manufacturer applies to the Model only. Though you can eager-load the manufacturer relationships from the listing object, see below.

class Listing extends Eloquent
{
    public function model()
    {
        return $this->belongsTo('Model', 'model_id');
    }
}

class Model extends Eloquent
{
    public function manufacturer()
    {
        return $this->belongsTo('manufacturer');
    }
}

class Manufacturer extends Eloquent
{
} 

$listings = Listing::with('model.manufacturer')->all();
foreach($listings as $listing) {
    echo $listing->model->name . ' by ' . $listing->model->manufacturer->name;
}

It took a bit of finagling, to get your requested solution working. The solution looks like this:

public function manufacturer()
{
    $instance = new Manufacturer();
    $instance->setTable('models');
    $query = $instance->newQuery();

    return (new BelongsTo($query, $this, 'model_id', $instance->getKeyName(), 'manufacturer'))
        ->join('manufacturers', 'manufacturers.id', '=', 'models.manufacturer_id')
        ->select(DB::raw('manufacturers.*'));
}

I started off by working with the query and building the response from that. The query I was looking to create was something along the lines of:

SELECT * FROM manufacturers ma
    JOIN models m on m.manufacturer_id = ma.id
WHERE m.id in (?)

The query that would be normally created by doing return $this->belongsTo('Manufacturer');

select * from `manufacturers` where `manufacturers`.`id` in (?)

The ? would be replaced by the value of manufacturer_id columns from the listings table. This column doesn't exist, so a single 0 would be inserted and you'd never return a manufacturer.

In the query I wanted to recreate I was constraining by models.id. I could easily access that value in my relationship by defining the foreign key. So the relationship became

return $this->belongsTo('Manufacturer', 'model_id');

This produces the same query as it did before, but populates the ? with the model_ids. So this returns results, but generally incorrect results. Then I aimed to change the base table that I was selecting from. This value is derived from the model, so I changed the passed in model to Model.

return $this->belongsTo('Model', 'model_id');

We've now mimic the model relationship, so that's great I hadn't really got anywhere. But at least now, I could make the join to the manufacturers table. So again I updated the relationship:

return $this->belongsTo('Model', 'model_id')
    ->join('manufacturers', 'manufacturers.id', '=', 'models.manufacturer_id');

This got us one step closer, generating the following query:

select * from `models` 
    inner join `manufacturers` on `manufacturers`.`id` = `models`.`manufacturer_id`
    where `models`.`id` in (?)

From here, I wanted to limit the columns I was querying for to just the manufacturer columns, to do this I added the select specification. This brought the relationship to:

return $this->belongsTo('Model', 'model_id') ->join('manufacturers', 'manufacturers.id', '=', 'models.manufacturer_id') ->select(DB::raw('manufacturers.*'));

And got the query to

select manufacturers.* from `models` 
    inner join `manufacturers` on `manufacturers`.`id` = `models`.`manufacturer_id`
    where `models`.`id` in (?)

Now we have a 100% valid query, but the objects being returned from the relationship are of type Model not Manufacturer. And that's where the last bit of trickery came in. I needed to return a Manufacturer, but wanted it to constrain by themodelstable in the where clause. I created a new instance of Manufacturer and set the table tomodels` and manually create the relationship.

It is important to note, that saving will not work.

$listing = Listing::find(1);
$listing->manufacturer()->associate(Manufacturer::create([]));
$listing->save();

This will create a new Manufacturer and then update listings.model_id to the new manufacturer's id.

Answer 2

I guess that this could help, it helped me:

class Car extends Model
{
    public function mechanical()
    {
        return $this->belongsTo(Mechanical::class);
    }
}

class CarPiece extends Model
{
    public function car()
    {
        return $this->belongsTo(Car::class);
    }

    public function mechanical()
    {
        return $this->car->mechanical();
    }
}

At least, it was this need that made me think of the existence of a belongsToThrough