How do I order a has_many through association in Ruby on Rails?

Given the following AR models, I would like to sort users alphabetically by last name when given a handle to a task:

#user
has_many :assignments
has_many :tasks, :through => :assignments    

#assignment
belongs_to :task
belongs_to :user

#task
has_many :assignments
has_many :users, :through => :assignments

I would like to get a task then navigation to its assigned users, and sort the user list alphabetically.

I keep thinking that I should be able to add the :order clause to has_many :users, :through => :assignments like this:

#task.rb
has_many :assignments
has_many :users, :through => :assignments, :order => 'last_name, first_name'

however this does not work.

How can I sort users by last_name when given a task?

Since condition arguments are deprecated in Rails 4, one should use scope blocks:

has_many :users, -> { order 'users.last_name, users.first_name' }, :through => :assignments

Rails 3.x version:

has_many :users, :through => :assignments, :order => 'users.last_name, users.first_name'

UPDATE: This only works in Rails 3.x (maybe before that too). For 4+, see other answers.