Filter on parent object attribute in ActiveAdmin

Question

I'd like to be able to filter an object based on an attribute of it's parent:

class Call < ActiveRecord::Base
  belongs_to :user
end

class User < ActiveRecord::Base
  has_many :calls
end

I'd like to be able to do this:

ActiveAdmin.register Call do
  filter :user
end

and have it filter on user.name, rather than present a select of all users. Can this be done?

Answer 1

Denis's solution almost worked for me. I just needed to add the filter type. For example:

ActiveAdmin.register Call do
  filter :user_name, :as => :string
end

Answer 2

In the next release of ActiveAdmin (I work with 1.0.0.pre) you can use Ransack methods. So, let say you have an Article, which belongs_to User.

You will have the following admin/article.rb file

ActiveAdmin.register Article do

  controller do
    def scoped_collection
      Article.includes(:user)
    end
  end  

  index do      
   column :id
   column :created_at
   column :title
   column("Author", sortable: 'users.first_name') { |item| link_to item.user.full_name, user_path(item.user) }
   actions
  end

  filter :user_first_name_cont, :as => :string
  filter :user_last_name_cont, :as => :string  

end

Here, user_first_name_cont is ransack method which filters on associated user first_name and 'cont' means contains.