Ruby on Rails 3：如何通过关联的方法查找关联的记录？

Question

I have Invoices with many Items and Payments.

This is my Invoice model (at least a part of it):

def total
  items.sum { |item| item.total }
end

def balance
  self.payments.sum(:amount) - self.total
end

Now in the Payment view I am trying to find all the Invoices with a balance less than 0:

<%= f.select(:invoice_id, current_user.invoices.where("balance > ?", 0)) %>

This doesn't work of course, because balance is not a column. But how can I insert a method there?

Thanks for any help.

Answer 1

You have a couple of options. You could always make balance a column (updating it as appropriate when payments or items are added), which will allow you do this sort of query efficiently.

You could, however, just use a plain Ruby approach - specifically, Array#select . Something like

current_user.invoices.to_a.select {|invoice| invoice.balance < 0}

would get you there. It wouldn't be very efficient if users have a large number of invoices though.

Update : also, you're going to hit an N+1 query situation if you do it like this, because each invoice will need to retrieve its items and payments from the database. To avoid this, you need to include the items and payments in the original query like so:

current_user.invoices.includes(:items, :payments).to_a.select{ ... }