Laravel - reverse polymorphic relationship

Question

I basically have two models (dog and cat) of the same type: pets. The table pets joins all dogs and cats in the database. Now I would like to be able to find a specific pet through the pet id in the PetController. Like so:

$pet = Pet::findOrFail($id); // returns a dog or cat

Tables structure:

┌──────────────┐  ┌───────────┐  ┌───────────┐
│ pets         │  │ dogs      │  │ cats      │
├──────────────┤  ├───────────┤  ├───────────┤
│ id           │  │ id        │  │ id        │
│ related_type │  │ name      │  │ name      │
│ related_id   │  │ eye_color │  │ tail_size │
└──────────────┘  └───────────┘  └───────────┘

Pets table:

┌────┬──────────────┬────────────┐
│ id │ related_type │ related_id │
├────┼──────────────┼────────────┤
│ 1  │ dog          │ 1          │
├────┼──────────────┼────────────┤
│ 2  │ dog          │ 2          │
├────┼──────────────┼────────────┤
│ 3  │ cat          │ 1          │
└────┴──────────────┴────────────┘

I have searched the Laravel docs but none of the relationships seem to fit for this problem. Only the polymorphic relationship would work the other way around, so that I could access the pet model through the dog- or cat-id. But I am looking for a solution that work the other way around. Is there any kind of relationship without needing to use nasty if-else in the PetController manually?

Thank you!

No i.e. I would like to find any type of pet with pet-id 3 (which returns cat of id 1) — pixelmusik, Jul 12 '18 at 11:00
I am looking for any kind of relationship that concatenate the cat or dog with the entry in pets. Since they are of different types I cannot simply use foreign keys. — pixelmusik, Jul 12 '18 at 11:06

rkj · Accepted Answer · 2018-07-13T13:24:18.120

12

You can define a polymorphic relationship between these 3 models like this

Pet Model

public function related(){
     $this->morphTo();
}

Dog Model

public function pets(){
     $this->morphMany('App\Pet', 'related');
}

Cat Model

public function pets(){
     $this->morphMany('App\Pet', 'related');
}

Now fetch it like this

$pet = Pet::findOrFail($id)->related;
dd($pet); //you will get either cat or dog

Easy create

$dog = Dog::create(['name'=> 'dog1', 'eye_color' => 'gray']);
$dog->pets()->create();

Check details here https://laravel.com/docs/5.6/eloquent-relationships#polymorphic-relations

edited Jul 13 '18 at 13:24

answered Jul 13 '18 at 13:04

rkj

8,067
2
27
33

1

Great answer! Though for this particular example, `morphOne` would be more appropriate than `morphMany` as one dog/cat row should have only one corresponding row in pets. – Vanlalhriata May 27 '20 at 11:52

score 4 · Answer 2 · edited Aug 23 '22 at 07:46

You can create your own trait for this:

app/MorphToModel.php

<?php

namespace App;

trait MorphToModel
{
    protected function morphToModel($related, $name = 'related', $foreignKey = 'id')
    {
        $table = $this->getTable();

        return $this->belongsTo($related, $name . '_id', $foreignKey)
            ->join(
                $table,
                $table . '.' . $name . '_id',
                ($model = new $related)->getTable() . '.' . $model->getKeyName()
            )
            ->select($model->getTable() . '.*')
            ->where($table . '.' . $name . '_type', $related);
    }
}

Use the trait in your model(s):

app/Pet.php

<?php

namespace App;

use Illuminate\Database\Eloquent\Model;

class Pet extends Model
{
    use MorphToModel;

    // ...

    public function related()
    {
        return $this->morphTo();
    }

    public function cat()
    {
        return $this->morphToModel(Cat::class);
    }

    public function dog()
    {
        return $this->morphToModel(Dog::class);
    }
}

Usage:

$pet = Pet::findOrFail($id);

$pet->cat; // A cat model or null.

$pet->dog; // A dog model or null.

$pet->cat() // A cat relationship query builder.

$pet->dog() // A dog relationship query builder.

One more point is to add "->select($model->getTable() . '.*')" — Yevhen Kotliar, Dec 07 '21 at 19:21

score 2 · Answer 3 · answered Jul 12 '18 at 11:15

2

You need to keep Model namespase in pats table(related_type column). Add to your PetModel

public function concretePet()
{
    return $this->hasOne($this->related_type, 'id', 'related_id');
}

Use:

$pet = Pet::findOrFail($id)->concretePet;

answered Jul 12 '18 at 11:15

J. Doe

1,682
11
22

Thank you, looks great! I will have a look at it! – pixelmusik Jul 12 '18 at 14:01
2

Unfortunately this does not work with eager loadings since `$this->related_type` is null. – pixelmusik Jul 17 '18 at 08:03

Laravel - reverse polymorphic relationship

3 Answers3