Fix config.parent_controller to work after the class loading

Fixes #2790, Refs. e4ae669

.rubocop.yml

@@ -108,7 +108,7 @@ Metrics/MethodLength:
   Max: 29 # TODO: Lower to 15
 
 Metrics/ModuleLength:
-  Max: 200 # TODO: Lower to 100
+  Max: 202 # TODO: Lower to 100
 
 Metrics/ParameterLists:
   Max: 8 # TODO: Lower to 4

lib/rails_admin/config.rb

@@ -67,7 +67,7 @@ class << self
       attr_accessor :sidescroll
 
       # set parent controller
-      attr_accessor :parent_controller
+      attr_reader :parent_controller
 
       # set settings for `protect_from_forgery` method
       # By default, it raises exception upon invalid CSRF tokens
@@ -270,6 +270,14 @@ def default_hidden_fields=(fields)
         end
       end
 
+      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
+      end
+
       # Returns action configuration object
       def actions(&block)
         RailsAdmin::Config::Actions.instance_eval(&block) if block

spec/rails_admin/config_spec.rb

@@ -262,6 +262,10 @@ class RecursivelyEmbedsMany
   end
 
   describe '.parent_controller' do
+    before do
+      class TestController < ActionController::Base; end
+    end
+
     it 'uses default class' do
       expect(RailsAdmin.config.parent_controller).to eq '::ActionController::Base'
     end
@@ -274,6 +278,21 @@ class RecursivelyEmbedsMany
     end
   end
 
+  describe '.parent_controller=' do
+    context 'if RailsAdmin::ApplicationController is already loaded' do
+      after do
+        RailsAdmin::Config.reset
+        RailsAdmin.send(:remove_const, :ApplicationController)
+        load RailsAdmin::Engine.root.join('app/controllers/rails_admin/application_controller.rb')
+      end
+
+      it 'can be changed' do
+        RailsAdmin.config.parent_controller = 'ApplicationController'
+        expect(RailsAdmin::ApplicationController.superclass).to eq ApplicationController
+      end
+    end
+  end
+
   describe '.forgery_protection_settings' do
     it 'uses with: :exception by default' do
       expect(RailsAdmin.config.forgery_protection_settings).to eq(with: :exception)