Finding unique records, ordered by field in association, with PostgreSQL and Rails 3?-Collection of common programming errors

UPDATE: So thanks to @Erwin Brandstetter, I now have this:

def self.unique_users_by_company(company)
  users = User.arel_table
  cards = Card.arel_table

  users_columns = User.column_names.map { |col| users[col.to_sym] }

  cards_condition = cards[:company_id].eq(company.id).
    and(cards[:user_id].eq(users[:id]))

  User.joins(:cards).where(cards_condition).group(users_columns).
    order('min(cards.created_at)')
end

… which seems to do exactly what I want. There are two shortcomings that I would still like to have addressed, however:

1. The order() clause is using straight SQL instead of Arel (couldn’t figure it out).

2. Calling .count on the query above gives me this error:

   NoMethodError: undefined method ‘to_sym’ for # from /Users/neezer/.rvm/gems/ruby-1.9.3-p0/gems/activerecord-3.1.1/lib/active_record/relation/calculations.rb:227:in ‘execute_grouped_calculation’

   … which I believe is probably related to how I’m mapping out the users_columns, so I don’t have to manually type in all of them in the group clause.

How can I fix those two issues?

ORIGINAL QUESTION:

Here’s what I have so far that solves the first part of my question:

def self.unique_users_by_company(company)
  users = User.arel_table
  cards = Card.arel_table

  cards_condition = cards[:company_id].eq(company.id)
    .and(cards[:user_id].eq(users[:id]))

  User.where(Card.where(cards_condition).exists)
end

This gives me 84 unique records, which is correct.

The problem is that I need those User records ordered by cards[:created_at] (whichever is earliest for that particular user). Appending .order(cards[:created_at]) to the scope at the end of the method above does absolutely nothing.

I tried adding in a .joins(:cards), but that give returns 587 records, which is incorrect (duplicate Users). group_by as I understand it is practically useless here as well, because of how PostgreSQL handles it.

I need my result to be an ActiveRecord::Relation (so it’s chainable) that returns a list of unique users who have cards that belong to a given company, ordered by the creation date of their first card… with a query that’s written in Ruby and is database-agnostic. How can I do this?

class Company
  has_many :cards
end

class Card
  belongs_to :user
  belongs_to :company
end

class User
  has_many :cards
end

Please let me know if you need any other information, or if I wasn’t clear in my question.