Select filter is not working

[kot-begemot]

Spec:

activeadmin (1.4.3)
activeadmin-mongoid (0.8.0)
  remote: https://github.com/activeadmin/activeadmin-mongoid.git
  revision: ff45b6379cec574e687548b607cad225fd2cf403
  branch: master
 ransack-mongoid (0.1.0)
  remote: https://github.com/activerecord-hackery/ransack-mongoid.git
  revision: bd1935293d2f285fa7cf33d2c5c90d981bf67338

Error
undefined method +' for nil:NilClass`

Reported line
activeadmin (1.4.3) lib/active_admin/inputs/filters/select_input.rb

Active admin setup:

ActiveAdmin.register SellOrder do
  actions :all, :except => [:new, :edit, :destroy]

  index do
    selectable_column
    column :name
    column :user
    column "Domains" do |sell_order|
      sell_order.domains.count
    end
    column :state
    actions
  end

  filter :name
  filter :state, as: :select, collection: SellOrder.state_machine.states.map(&:name)
  filter :created_at

  show do
     ...
  end
end

Investigation
activeadmin-mongoid substitutes method searchable_method_name with broken equivalent, while original method itself works like a charm

[grzegorz-jakubiak]

Would you be willing to submit a PR?

[kot-begemot]

Sure, I'll do

[grzegorz-jakubiak]

@kot-begemot please try this #143