How to use the relationship with BelongsToMany in Eloquent

I'm having trouble how to use the belongsToMany method of Laravel to create a relationship. I don't fully understand the description of the parameters. I have the following situation:

Fields of table products:

id
...
product_unique_identifier

Fields of table product_images:

id
app_image_id
product_unique_identifier

Fields of table app_images:

id
...

Now as you can imagine, I want to use in my Product model as well as in my AppImage model a relationship to the other table using the product_images table. The thing is, that the column product_unique_identifier is not the identity field in the products table, but a field of type string.

One product can have multiple images - one image is always related to one product.

I already tried this in my AppImage model:

public function product() {
    return $this->belongsToMany(\App\Models\Product\Product::class, 'product_images','app_image_id','product_unique_key','id','product_unique_key');
}

This is not working as expected. I already had a look into the Laravel framework code (https://github.com/laravel/framework/blob/9.x/src/Illuminate/Database/Eloquent/Relations/BelongsToMany.php), but the explanations of the parameters is still not clear to me (which are pivot and related fields as described in the comments of the properties).

Solution

Try this

public function product() {
    return $this->belongsToMany(\App\Models\Product\Product::class, 'product_images','app_image_id','product_unique_identifier');
}