RoR/Postgres Query for the first or last child object of all parents - postgresql

Question

I have seen similar questions, but no answers that match exactly what I need. If it's out there, I haven't seen it. I'm on Rails 3.1, Ruby 1.9.2, deployed on Heroku Cedar stack (so I'm using postgres)
I've got this model:
class User
has_many :orders
end
class Order
belongs_to :user
#scope :last_for_each_user, ???
#scope :first_for_each_user, ???
end
I need scopes (or methods) that return all orders that are all users' first orders, and all orders that are all users' last orders. My first instinct was to use GROUP BY, which worked fine for :last_for_each_user in SQLite3, but Postgres won't let you select columns that aren't in the GROUP BY clause or selected as part of aggregate functions, and I need fully formed orders objects (i.e., all of the columns, orders.*).
Any ideas? I don't want to select the whole table and sort it out in Ruby. That just doesn't feel right, and it's a lot to hold in memory.

Answer 1

So here's what I ended up doing, for future head-scratchers:
scope :last_for_each_user, lambda {
select("orders.*") & Order.select("max(orders.id) as id").group("orders.user_id")
}
scope :first_for_each_user, lambda {
select("orders.*") & Order.select("min(orders.id) as id").group("orders.user_id")
}
If anyone has a better idea, I am very much open to hear it. And thanks to the whole SO community (even though no one responded), this is the first time I've signed up/asked a question but every time I google a problem, I end up here!

Answer 2

You could use the DISTINCT ON() rules to get only the last one. Here's an example following your model structure:
scope :for_users, lambda {|ids, is_first| includes(:user).where("user_id IN (?)", ids).select("DISTINCT ON (users.id) *").order("orders.created_at ?", is_first ? : "ASC" : "DESC") }
scope :last_for_each_users, lambda { |users| for_each_users(users.map{|u| u.id }, false }
scope :first_for_each_users, lambda { |users| for_each_users(users.map{|u| u.id }, true }