How can I calculate sum of query results in Elixir?

Question

I have an order table. One of columns of this table is total price.

Total price is calculated by adding up all prices of order_item and item_attribute

When a user deletes an order_item, I want to make it delete all of related item_attribute to order_item. So when it calculates total price, it should deduct price of order_item and item_attribute.

So to do that I made code like below

order_item_total = Repo.one!(from p in Pos8.OrderItem, where: p.order_id == ^order_id, select: sum(p.item_total))
total_item_attributes_price = Repo.one!(from p in Pos8.ItemAttribute, where: p.order_id == ^order_id, select: sum(p.price))

order_total_query = (order_item_total + total_item_attributes_price)

Pos8.Order.changeset(order, %{total: order_total_query})
|> Repo.insert_or_update

After delete order_item and item_attribute, it selects sum of total price of order_item and item_attribute, which are related to order_id. And then input its value to Order database.

However, it triggers (ArithmeticError) bad argument in arithmetic expression. Which shows that order_total_query = (order_item_total + total_item_attributes_price) is not valid argument in arithmetic expression...

How should I calculate order_total_queryin correct arithmetic expression?

Thanks in advance.

Answer 1

How about using Repo.aggregate/4 ?