Instance variables and do loops in ruby on rails

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?

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 = ?;

StackExchange Code Review Q#13130, answer score: 6