restore 2.x config loading behavior

Attempt to restore 2.x config loading behavior where 1) per-model config blocks
only evaluate after all the models are loaded and associations are defined
while 2) initializer config block evaluates immediately so that routes are
configured correctly.

One remaining change compared to 2.x is RailsAdmin::Config#model no longer
returns the model when given a block. With the removal of LazyModel in e4ae669
it is tricky to both defer initialization and return the model class.

Fixes railsadminteam#3490

.rubocop.yml

@@ -118,7 +118,7 @@ Metrics/MethodLength:
   Max: 29 # TODO: Lower to 15
 
 Metrics/ModuleLength:
-  Max: 217 # TODO: Lower to 100
+  Max: 228 # TODO: Lower to 100
 
 Metrics/ParameterLists:
   Max: 8 # TODO: Lower to 4

lib/rails_admin.rb

@@ -29,7 +29,9 @@ def self.config(entity = nil, &block)
     if entity
       RailsAdmin::Config.model(entity, &block)
     elsif block_given?
-      RailsAdmin::Config.apply(&block)
+      # Immediately evaluate non-model (initializer) config. It's needed to
+      # properly configure routes when there are custom actions.
+      yield RailsAdmin::Config
     else
       RailsAdmin::Config
     end

lib/rails_admin/adapters/mongoid/extension.rb

@@ -11,9 +11,7 @@ module Extension
           self.nested_attributes_options = {}
           class << self
             def rails_admin(&block)
-              RailsAdmin.config do |config|
-                config.model(self, &block)
-              end
+              RailsAdmin.config(self, &block)
             end
 
             alias_method :accepts_nested_attributes_for_without_rails_admin, :accepts_nested_attributes_for

lib/rails_admin/config.rb

@@ -24,6 +24,7 @@ module Config
 
     # Variables to track initialization process
     @initialized = false
+    @initializing = false
     @deferred_blocks = []
 
     class << self
@@ -87,6 +88,9 @@ class << self
 
       # Finish initialization by executing deferred configuration blocks
       def initialize!
+        return if @initializing
+
+        @initializing = true
         @deferred_blocks.each { |block| block.call(self) }
         @deferred_blocks.clear
         @initialized = true
@@ -226,6 +230,7 @@ def default_search_operator=(operator)
 
       # pool of all found model names from the whole application
       def models_pool
+        initialize!
         (viable_models - excluded_models.collect(&:to_s)).uniq.sort
       end
 
@@ -238,7 +243,7 @@ def models_pool
       #
       # If a block is given it is evaluated in the context of configuration instance.
       #
-      # Returns given model's configuration
+      # Otherwise returns given model's configuration.
       #
       # @see RailsAdmin::Config.registry
       def model(entity, &block)
@@ -254,9 +259,24 @@ def model(entity, &block)
             entity.class.name.to_sym
           end
 
-        @registry[key] ||= RailsAdmin::Config::Model.new(entity)
-        @registry[key].instance_eval(&block) if block && @registry[key].abstract_model
-        @registry[key]
+        # When a block is provided defer evaluation until initialize so that:
+        #
+        # 1) performing configuration in the initializer does not attempt to
+        #    autoload model classes, and
+        # 2) model config blocks run after models are fully loaded and
+        #    associations are defined.
+        #
+        # Do not defer when called without a block as the model must be returned.
+        if block
+          RailsAdmin::Config.apply do
+            m = model(entity)
+            m.instance_eval(&block) if @registry[key].abstract_model
+          end
+          nil
+        else
+          initialize!
+          @registry[key] ||= RailsAdmin::Config::Model.new(entity)
+        end
       end
 
       def asset_source
@@ -318,6 +338,8 @@ def models
       #
       # @see RailsAdmin::Config.registry
       def reset
+        @initialized = @initializing = false
+        @deferred_blocks.clear
         @compact_show_view = true
         @browser_validations = true
         @authenticate = nil
@@ -356,10 +378,8 @@ def reset_model(model)
 
       # Perform reset, then load RailsAdmin initializer again
       def reload!
-        @initialized = false
         reset
         load RailsAdmin::Engine.config.initializer_path
-        initialize!
       end
 
       # Get all models that are configured as visible sorted by their weight and label.

lib/rails_admin/engine.rb

@@ -68,11 +68,6 @@ class Engine < Rails::Engine
         ERROR
       end
 
-      RailsAdmin::Config.initialize!
-
-      # Force route reload, since it doesn't reflect RailsAdmin action configuration yet
-      app.reload_routes!
-
       RailsAdmin::Version.warn_with_js_version
     end
   end

spec/rails_admin/config_spec.rb

@@ -327,6 +327,24 @@ class TestController < ActionController::Base; end
       end
     end
 
+    context 'when a block is not provided' do
+      let(:model) { described_class.model(Team) }
+      it 'returns the model config' do
+        expect(model).to be_a(RailsAdmin::Config::Model)
+      end
+    end
+
+    context 'when a block is provided' do
+      let(:model) do
+        described_class.model(Team) do
+          field :fans
+        end
+      end
+      it 'does not return the model config' do
+        expect(model).to be_nil
+      end
+    end
+
     context 'when model expanded' do
       before do
         described_class.model(Team) do
@@ -387,6 +405,7 @@ class TestController < ActionController::Base; end
       end
     end
     before { RailsAdmin::Config.instance_variable_set(:@initialized, false) }
+    before { RailsAdmin::Config.instance_variable_set(:@initializing, false) }
     after do
       RailsAdmin::Config.instance_variable_set(:@initialized, true)
       RailsAdmin::Config.instance_variable_set(:@deferred_blocks, [])
@@ -445,6 +464,11 @@ class TestController < ActionController::Base; end
       expect(RailsAdmin::Config.model(Team).fields.find { |f| f.name == :color }.type).to eq :color
     end
 
+    it 'does not immediately apply model configuration' do
+      expect(RailsAdmin::Config).not_to receive(:initialize!)
+      RailsAdmin::Config.reload!
+    end
+
     it "applies the initializer's configuration first, then models' configurations" do
       # simulate the situation that Team model is loaded in the middle of processing RailsAdmin initializer
       allow_any_instance_of(RailsAdmin::Config::Model).to receive(:include_all_fields).and_wrap_original do |method|