Fix inheritance of parent_controller when controllers were eagerly loaded

Author: jalyna

lib/rails_admin/config.rb

@@ -269,10 +269,16 @@ def default_hidden_fields=(fields)
 
       def parent_controller=(name)
         @parent_controller = name
+
         if defined?(RailsAdmin::ApplicationController)
           RailsAdmin.send(:remove_const, :ApplicationController)
           load RailsAdmin::Engine.root.join('app/controllers/rails_admin/application_controller.rb')
         end
+
+        if defined?(RailsAdmin::MainController)
+          RailsAdmin.send(:remove_const, :MainController)
+          load RailsAdmin::Engine.root.join('app/controllers/rails_admin/main_controller.rb')
+        end
       end
 
       def total_columns_width=(_)

spec/rails_admin/config_spec.rb

@@ -283,6 +283,11 @@ class TestController < ActionController::Base; end
 
   describe '.parent_controller=' do
     context 'if RailsAdmin::ApplicationController is already loaded' do
+      before do
+        # preload cotrollers (e.g. when config.eager_load = true)
+        RailsAdmin::MainController
+      end
+
       after do
         RailsAdmin::Config.reset
         RailsAdmin.send(:remove_const, :ApplicationController)
@@ -292,6 +297,7 @@ class TestController < ActionController::Base; end
       it 'can be changed' do
         RailsAdmin.config.parent_controller = 'ApplicationController'
         expect(RailsAdmin::ApplicationController.superclass).to eq ApplicationController
+        expect(RailsAdmin::MainController.superclass.superclass).to eq ApplicationController
       end
     end
   end