Show the warning message if asset_source is not set

Refs. #3467

.rubocop.yml

@@ -118,7 +118,7 @@ Metrics/MethodLength:
   Max: 29 # TODO: Lower to 15
 
 Metrics/ModuleLength:
-  Max: 207 # TODO: Lower to 100
+  Max: 217 # TODO: Lower to 100
 
 Metrics/ParameterLists:
   Max: 8 # TODO: Lower to 4

lib/rails_admin/config.rb

@@ -81,7 +81,7 @@ class << self
       attr_accessor :navigation_static_label
 
       # Set where RailsAdmin fetches JS/CSS from, defaults to :sprockets
-      attr_accessor :asset_source
+      attr_writer :asset_source
 
       # Finish initialization by executing deferred configuration blocks
       def initialize!
@@ -257,6 +257,17 @@ def model(entity, &block)
         @registry[key]
       end
 
+      def asset_source
+        @asset_source ||=
+          begin
+            warn <<-MSG.gsub(/^ +/, '').freeze
+              [Warning] After upgrading RailsAdmin to 3.x you haven't set asset_source yet, using :sprockets as the default.
+              To suppress this message, run 'rails rails_admin:install' to setup the asset delivery method suitable to you.
+            MSG
+            :sprockets
+          end
+      end
+
       def default_hidden_fields=(fields)
         if fields.is_a?(Array)
           @default_hidden_fields = {}
@@ -326,7 +337,7 @@ def reset
         @show_gravatar = true
         @navigation_static_links = {}
         @navigation_static_label = nil
-        @asset_source = (defined?(Webpacker) ? :webpacker : :sprockets)
+        @asset_source = nil
         @parent_controller = '::ActionController::Base'
         @forgery_protection_settings = {with: :exception}
         RailsAdmin::Config::Actions.reset

rails_admin.gemspec

@@ -25,7 +25,7 @@ Gem::Specification.new do |spec|
   spec.summary = 'Admin for Rails'
   spec.version = RailsAdmin::Version
   spec.post_install_message = '
-    ### Upgrading from 2.x.x to 3.x.x ###
+    ### Upgrading RailsAdmin from 2.x.x to 3.x.x ###
 
     Due to introduction of Webpack/Webpacker support, some additional dependencies and configuration will be needed.
     Running `bin/rails g rails_admin:install` will suggest required changes, based on the current setup of your app.

spec/spec_helper.rb

@@ -105,6 +105,7 @@ def password_digest(password)
 
     DatabaseCleaner.start
     RailsAdmin::Config.reset
+    RailsAdmin::Config.asset_source = CI_ASSET
   end
 
   config.after(:each) do