Take quantity of an order and the price of the objetcs that belongs to, to show the total in Rails 6

I have the object Tool with a price and a ToolsOrder for every sale with a quantity. I need to show total of the sales

Tool has many: tools_orders
ToolsOrders belongs_to: tool

I think in something like this or similar? I can’t make it work

ToolsOrder.sum("quantity * Tool.price")

How can I calculate the total sum?

Answer

Sometimes it’s useful to read error messages that the code gives you. Assuming the error you got was: missing FROM-clause entry for table "tool". This is saying that you’re querying for price from table tool, but there is no such table in the current query.

You’re missing a join.

ToolsOrder.joins(:tool).sum("quantity * tools.price")

It can be made safer using Arel though if you’d like.

ToolsOrder.joins(:tool).sum(ToolsOrder.arel_table[:quantity] * Tool.arel_table[:price])

Now there’s not pure sql, that is easy to make typos in (Tool vs tools) and is open to sql injections.