[php] Laravel Eloquent Sum of relation's column

I've been working on a shopping cart application and now I've come to the following issue..

There is a User, a Product and a Cart object.

• The Cart table only contains the following columns: id, user_id, product_id and timestamps.
• The UserModel hasMany Carts (because a user can store multiple products).
• The CartModel belongsTo a User and CartModel hasMany Products.

Now to calculate the total products I can just call: Auth::user()->cart()->count().

My question is: How can I get the SUM() of prices (a column of product) of the products in cart by this User?
I would like to accomplish this with Eloquent and not by using a query (mainly because I believe it is a lot cleaner).

Also using query builder

DB::table("rates")->get()->sum("rate_value")

To get summation of all rate value inside table rates.

To get summation of user products.

DB::table("users")->get()->sum("products")

this is not your answer but is for those come here searching solution for another problem. I wanted to get sum of a column of related table conditionally. In my database Deals has many Activities I wanted to get the sum of the "amount_total" from Activities table where activities.deal_id = deal.id and activities.status = paid so i did this.

$query->withCount([
'activity AS paid_sum' => function ($query) {
            $query->select(DB::raw("SUM(amount_total) as paidsum"))->where('status', 'paid');
        }
    ]);

it returns

"paid_sum_count" => "320.00"

in Deals attribute.

This it now the sum which i wanted to get not the count.

I tried doing something similar, which took me a lot of time before I could figure out the collect() function. So you can have something this way:

collect($items)->sum('amount');

This will give you the sum total of all the items.

you can do it using eloquent easily like this

$sum = Model::sum('sum_field');

its will return a sum of fields, if apply condition on it that is also simple

$sum = Model::where('status', 'paid')->sum('sum_field');