ActiveAdmin- adding a filter on has_one through association

Given Project, Task, and Activity, I've managed to set up a has_one :through relationship filter with the following:

class Project < ActiveRecord::Base
  has_many :tasks
end

class Task < ActiveRecord::Base
  belongs_to :project
  has_many :activities, :dependent => :destroy
end

class Activity < ActiveRecord::Base
  has_many :tasks
  has_one  :project, through: :task
end

ActiveAdmin.register User do
  filter :task_project_id, as: :select, collection: Project.all, label: 'Project'
end