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

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

Answer 3

You can do something like this (Student Group -> Users -> Poll results):
// poll result
public function studentGroup(): HasOneDeep
{
return $this->hasOneDeepFromRelations($this->user(), (new User())->studentGroup());
}