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

Question

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 :ta         


        

Answer 1

The M.G.Palmer's approach works well, however table name is involved. There is a better way to do it:

has_many :users, :through => :assignments, :order => [ :last_name, :first_name ]

Answer 2

This works for me (Rails 4.2)

Applying an ordering on the through map is not preserved, aka this is not enough to have the genres ordered:

has_many    :disk_genre_maps,
            -> {order('disk_genre_map.sort_order')},
            :inverse_of => :disk,
            :dependent  => :destroy,
            :autosave   => true


has_many    :genres,    # not sorted like disk_genre_maps
            :through    => :disk_genre_maps,
            :source     => :genre,
            :autosave   => true

So I overwrite this per instance:

def genres  # redefine genres per instance so that the ordering is preserved
    self.disk_genre_maps.map{|dgm|dgm.genre}
end

To make that work for assignments, this should be something like this (untested)

def genres= some_genres
    self.disk_genre_maps = some_genres.map.with_index do |genre, index|
        DiskGenreMap.new(disk:self, genre:genre, sort_order:index)
    end
end