Multi level sort with associations

Question

My setup: Rails 3.0.9, Ruby 1.9.2

Here are my models

class Project
  has_many :tasks
  has_many :photos

class Task
  belongs_to :project

class Photos
  belongs_to :project

What I need is to be able to return a result set sorted first by project name (ASC) and then interleave tasks and photos sorted by date (DESC). Here's a sample output of the order I need

project: install new roof
  photo: ...(added on 7/15/11)
  task: ...(added on 7/10/11)
  task: ...(added on 7/1/11)

project: repair bathtub
  task: ... (added on 8/21/11)
  photo: ... (added on 8/20/11)
  photo: ...(added on 8/17/11)
  task: ... (added on 8/15/11)

I prefer to get the entire result set from ActiveRecord and then sort it in Ruby but unsure how to go about it, any insights will be much appreciated. Thanks.

Answer 1

How about something like this? Grab all the objects, eagerly loaded from the DB, then sort them with Array#sort! .

# First, grab all projects and their task/photos from the DB
projects = Project.includes(:tasks, :photos).order("project_name ASC")

# Now, iterate over projects, combine tasks/photos, sort by date
projects.each do |p|
  tasks_and_photos = p.tasks + p.photos
  tasks_and_photos.sort! { |x,y| y.created_at <=> x.created_at }
  # now do something with the tasks and photos!
end

Adding a note to this, if you didn't want to use the created_at attribute for your sort condition, all you would need to do is ensure that both Photo and Task have some attribute with a common name, such as date_added or whatever. As long as there is something with a common name to sort by, you're good to go.