ActiveAdmin: sort by child association's property

Question

I am having these objects:

class District < ActiveRecord::Base
  belongs_to :city
end
class City < ActiveRecord::Base
  has_many :districts
end

What I would like to do (and been unable to do so thus far), is: have a City column in District's index and that column should be sortable on City.name.

Closest thing I was able to do without crashing ActiveAdmin is:

index do
  column City.human_name(:count => :other), :city, :sortable => :city_id
end

Which of course is not good enough; I don't want to sort on foreign key's integer value.

Tried stuff like :sortable => 'city.name', gives an error. Even tried to do it like you do it on "pure" Rails - :joins => :cities, :sortable => 'city.name' - no luck. Tried a bunch of other stupid stuff, got annoyed and decided to humbly ask for help.

Can anyone point me in the right direction? Thanks for your time.

Answer 1

That should also do the work:

index do
  column City.model_name.human, :city, :sortable => 'cities.name'
end

controller do
  def scoped_collection
    end_of_association_chain.includes(:city)
  end
end