ActiveAdmin filter, for a has_many :through association

active_admin_filter_for_has_many_through.rb

# app/models/user.rb
class User < ActiveRecord::Base
  has_many :permissions
  has_many :roles, through: :permissions

  scope :by_role, lambda { |role_ids|
    joins(:permissions).where(permissions: { role_id: Array.wrap(role_ids) })
  }
  
  ransacker :role,
            formatter: proc { |role_ids|
              matches = User.by_role(role_ids).map(&:id)
              matches.any? ? matches : nil
            } do |parent|
    parent.table[:id]
  end
end

# app/models/role.rb
class Role < ActiveRecord::Base
  has_many :permissions
  has_many :users, through: :permissions

  validates :name, uniqueness: true
end

# app/models/permission.rb
class Permission < ActiveRecord::Base
  belongs_to :role, counter_cache: :users_count
  belongs_to :user
end

# admin/user.rb
ActiveAdmin.register User do
  filter :role_in_all, as: :select, 
                       multiple: true,
                       collection: -> { Role.order(name: :asc).all }
end