Adding by block in ruby

Question

I posted a similar question to this not too long ago in regards to formatting a MySQL query using a block and got very good responses, but they were very specific to the problem at hand. This time around, I'm dealing with getting the .sum()s of rows in a table. Here's what I've got now:

def balance
  balance = 0
  items.each do |item|
    balance = balance + item.charges.sum(:revenue, :conditions => ['created_at >= ?', Time.now.beginning_of_month])
  end
  balance
end

My goal here is to get the total of all charges for this month for a given user. Charges belong to items, which belong users. I'm sure there's a better way to do this in Ruby/Rails.

What would you do?

Answer 1

A straight conversion can be done:

def balance
  conds = ["created_at > ?", Time.now.beginning_of_month]
  items.inject(0) do |total, item|
    total + item.charges.sum(:revenue, :conditions => conds)
  end
end

There might be more optimal approaches depending upon how your relationships are mapped out. For instance, you may be able to do something like:

def balance
  Charge.sum :revenue,
    :conditions => ["charges.item_id IN (?) AND created_at > ?",
      items.map { |item| item.id },
      Time.now.beginning_of_month]
end

For these kinds of situations, map, inject, select, and so forth, are invaluable tools. Here's a lengthy discussion on inject, and definitely consult the RDoc's for the Enumerable module for more information.

Answer 2

There's no reason not to do this in the SQL query itself, e.g:

Charges.sum :revenue, :conditions => [ "created_at >= ?, items.user_id = ?",
                                        Time.now.beginning_of_month, some_user_id ],
                      :joins => :items

Edit: It's unclear from the docs whether sum will take a symbol for :joins like find does. If it doesn't, your :joins line should look like this instead:

:joins => "JOIN items ON charges.item_id = items.id"

Answer 3

Just a couple of general comments:

• I would suggest putting your code in the model rather than a controller, view, or helper, following the Skinny Controller, Fat Model idea.
• Instead of writing balance = balance + item.charges.sum(:revenue, :conditions => ['created_at >= ?', Time.now.beginning_of_month]) you can use += and do: balance += item.charges.sum(:revenue, :conditions => ['created_at >= ?', Time.now.beginning_of_month]).
• It's not technically a problem, but it seems bad form to have a variable the same name as your method (i.e., 'balance').
• I would store Time.now.beginning_of_month in a variable outside your loop so it doesn't get recalculated every time.

Answer 4

This is a very common recursion pattern. It is called a catamorphsim in category theory, a fold in mathematics and functional programming, it is also sometimes called reduce and in Smalltalk it is called inject:into:. In Ruby, it is called inject or reduce (these two methods are aliases).

The idea is that you have a collection of values and you want to "reduce" or "fold" that collection of multiple values into a single value. (The Smalltalk name inject:into: comes from the fact that you inject a starting value into a block which is called for each element of the collection.)

def balance
  this_month = Time.now.beginning_of_month
  items.reduce(0) { |balance, item|
    balance + item.charges.sum(:revenue, :conditions => ['created_at >= ?', this_month])
  }
end