How to sum all properties of a nested collection?

Question

Given I got User.attachments and Attachment.visits as an integer with the number count.

How can I easily count all the visits of all images of that user?

Answer 1

Use ActiveRecord::Base#sum:

user.attachments.sum(:visits)

This should generate an efficient SQL query like this:

SELECT SUM(attachments.visits) FROM attachments WHERE attachments.user_id = ID

Answer 2

user.attachments.map{|a| a.visits}.sum