GroupBy results from a HasManyThrough a ManyToMany relationship in Laravel 5.5 - mysql

Question

I have a complex relationship like a ManyToMany relationship between Category and Products and a Product can have many Inventory listings from different Shops(user). A Shop can have multiple listing of the same Product as a variant(color/size).
Everything is fine till now!
The issue begins when showing the listing to the visitors. I want to show all listings under a category but don't want to show multiple listings of the same product from the same shop. Instead, want to pick the lowest sale_price listing from the shop.
I have three models and relations are like:
class Category extends Model
{
public function products()
{
return $this->belongsToMany(Product::class);
}
}
class Product extends Model
{
public function categories()
{
return $this->belongsToMany(Category::class);
}
public function inventories()
{
return $this->hasMany(Inventory::class);
}
}
class Inventory extends Model
{
public function product()
{
return $this->belongsTo(Product::class);
}
}
Tables:
//Categories
Schema::create('categories', function (Blueprint $table) {
$table->increments('id');
$table->string('name')->unique();
});
//Products
Schema::create('products', function (Blueprint $table) {
$table->increments('id');
$table->string('name')->unique();
});
//Pivot table
Schema::create('category_product', function (Blueprint $table) {
$table->integer('category_id');
$table->integer('product_id');
});
//Inventories
Schema::create('inventories', function (Blueprint $table) {
$table->increments('id');
$table->integer('shop_id');
$table->integer('product_id');
$table->string('sku', 200);
$table->string('title');
$table->decimal('sale_price', 10, 2);
});
As Laravel doesn't provide any manyToManyThough() relationship. I have added a listings method in the Category model. This method returns all listings:
public function listings()
{
$product_ids = $this->products()->pluck('products.id')->all();
return Inventory::whereIn('product_id', $product_ids)->paginate(20);
}
Then I tried this:
public function listings()
{
$product_ids = $this->products()->pluck('products.id')->all();
return Inventory::whereIn('product_id', $product_ids)->groupBy('shop_id','product_id')->paginate(20);
}
These methods produce MySQL GroupBy Error. Yes, I can filter the result after taking all result but that'll affect the pagination. How to get the sorted results and also keep the pagination ability. Thanks and day by day I got more indebted to the community. :)

Answer 1

You can create a direct relationship by "skipping" the products table:
public function listings() {
return $this->belongsToMany(
Inventory::class,
'category_product',
null,
'product_id',
null,
'product_id'
);
}