Instance variables and do loops in ruby on rails

Question

I find my self often doing things like this in Ruby on Rails:

@count = 0

LineItem.all.each do |item|
     if (item.cart_id == @cart.id)
       @count += item.quantity
     end
end

So many things look wrong with this, but I want to initialize my variable (@count) before the loop, and then be able to use it after I'm out of the loop.

Is there a better way to do this?

Answer 1

From what I can see, you are just collecting the item.quantity of all matching items. This can be accomplished by

@count = LineItem.all.find_all{|i| i.cart_id == @cart.id}.map{|i| i.quantity}.inject(0,:+)

Do you mean that you might be doing something else with in the cart_id check?

Answer 2

If you work with database you can do this more efficiently with sql request:

LineItem.where(:card_id => @card.id).sum('quantity')

This is equivalent sql query

SELECT SUM(items.identity) FROM items WHERE items.card_id = ?;