Load constants lazily on RailsAdmin initialization

config/initializers/active_record_extensions.rb

@@ -4,9 +4,7 @@
   module ActiveRecord
     class Base
       def self.rails_admin(&block)
-        RailsAdmin.config do |config|
-          config.model(self, &block)
-        end
+        RailsAdmin.config(self, &block)
       end
 
       def rails_admin_default_object_label_method

lib/rails_admin.rb

@@ -3,6 +3,7 @@
 require 'rails_admin/engine'
 require 'rails_admin/abstract_model'
 require 'rails_admin/config'
+require 'rails_admin/config/const_load_suppressor'
 require 'rails_admin/extension'
 require 'rails_admin/extensions/cancancan'
 require 'rails_admin/extensions/pundit'
@@ -12,6 +13,8 @@
 require 'yaml'
 
 module RailsAdmin
+  extend RailsAdmin::Config::ConstLoadSuppressor
+
   # Setup RailsAdmin
   #
   # Given the first argument is a model class, a model class name
@@ -29,7 +32,7 @@ def self.config(entity = nil, &block)
     if entity
       RailsAdmin::Config.model(entity, &block)
     elsif block_given?
-      RailsAdmin::Config.apply(&block)
+      suppress_const_load { 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

@@ -1,6 +1,6 @@
 # frozen_string_literal: true
 
-require 'rails_admin/config/model'
+require 'rails_admin/config/lazy_model'
 require 'rails_admin/config/sections/list'
 require 'active_support/core_ext/module/attribute_accessors'
 
@@ -22,10 +22,6 @@ module Config
 
     DEFAULT_CURRENT_USER = proc {}
 
-    # Variables to track initialization process
-    @initialized = false
-    @deferred_blocks = []
-
     class << self
       # Application title, can be an array of two elements
       attr_accessor :main_app_name
@@ -88,22 +84,6 @@ class << self
       # Set where RailsAdmin fetches JS/CSS from, defaults to :sprockets
       attr_writer :asset_source
 
-      # Finish initialization by executing deferred configuration blocks
-      def initialize!
-        @deferred_blocks.each { |block| block.call(self) }
-        @deferred_blocks.clear
-        @initialized = true
-      end
-
-      # Evaluate the given block either immediately or lazily, based on initialization status.
-      def apply(&block)
-        if @initialized
-          yield(self)
-        else
-          @deferred_blocks << block
-        end
-      end
-
       # Setup authentication to be run as a before filter
       # This is run inside the controller instance so you can setup any authentication you need to
       #
@@ -249,27 +229,30 @@ def model(entity, &block)
           case entity
           when RailsAdmin::AbstractModel
             entity.model.try(:name).try :to_sym
-          when Class
+          when Class, ConstLoadSuppressor::ConstProxy
             entity.name.to_sym
           when String, Symbol
             entity.to_sym
           else
             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] ||= RailsAdmin::Config::LazyModel.new(key.to_s)
+        @registry[key].add_deferred_block(&block) if block
         @registry[key]
       end
 
       def asset_source
         @asset_source ||=
           begin
-            warn <<~MSG
-              [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
+            detected = defined?(Sprockets) ? :sprockets : :invalid
+            unless ARGV.join(' ').include? 'rails_admin:install'
+              warn <<~MSG
+                [Warning] After upgrading RailsAdmin to 3.x you haven't set asset_source yet, using :#{detected} as the default.
+                To suppress this message, run 'rails rails_admin:install' to setup the asset delivery method suitable to you.
+              MSG
+            end
+            detected
           end
       end
 
@@ -360,10 +343,8 @@ 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/config/const_load_suppressor.rb

@@ -0,0 +1,50 @@
+# frozen_string_literal: true
+
+module RailsAdmin
+  module Config
+    module ConstLoadSuppressor
+      def suppress_const_load
+        original = Object.method(:const_missing)
+        Object.define_singleton_method(:const_missing) do |name|
+          ConstProxy.new(name.to_s)
+        end
+        yield
+      ensure
+        Object.define_singleton_method(:const_missing, original)
+      end
+
+      class ConstProxy < BasicObject
+        attr_reader :name
+
+        def initialize(name)
+          @name = name
+        end
+
+        def klass
+          @klass ||=
+            begin
+              unless ::Object.const_defined?(name)
+                ::Kernel.raise <<~MESSAGE
+                  The constant #{name} is not loaded yet upon the execution of the RailsAdmin initializer.
+                  We don't recommend to do this and may lead to issues, but if you really have to do so you can explicitly require it by adding:
+
+                    require '#{name.underscore}'
+
+                  on top of config/initializers/rails_admin.rb.
+                MESSAGE
+              end
+              name.constantize
+            end
+        end
+
+        def method_missing(method_name, *args, &block)
+          klass.send(method_name, *args, &block)
+        end
+
+        def respond_to_missing?(method_name, include_private = false)
+          super || klass.respond_to?(method_name, include_private)
+        end
+      end
+    end
+  end
+end

lib/rails_admin/config/lazy_model.rb

@@ -0,0 +1,74 @@
+# frozen_string_literal: true
+
+require 'rails_admin/config/model'
+
+module RailsAdmin
+  module Config
+    class LazyModel < BasicObject
+      def initialize(entity, &block)
+        @entity = entity
+        @deferred_blocks = [*block]
+        @initialized = false
+      end
+
+      def add_deferred_block(&block)
+        if @initialized
+          @model.instance_eval(&block)
+        else
+          @deferred_blocks << block
+        end
+      end
+
+      def target
+        @model ||= ::RailsAdmin::Config::Model.new(@entity)
+        # When evaluating multiple configuration blocks, the order of
+        # execution is important. As one would expect (in my opinion),
+        # options defined within a resource should take precedence over
+        # more general options defined in an initializer. This way,
+        # general settings for a number of resources could be specified
+        # in the initializer, while models could override these settings
+        # later, if required.
+        #
+        # CAVEAT: It cannot be guaranteed that blocks defined in an initializer
+        # will be loaded (and adde to @deferred_blocks) first. For instance, if
+        # the initializer references a model class before defining
+        # a RailsAdmin configuration block, the configuration from the
+        # resource will get added to @deferred_blocks first:
+        #
+        #     # app/models/some_model.rb
+        #     class SomeModel
+        #       rails_admin do
+        #         :
+        #       end
+        #     end
+        #
+        #     # config/initializers/rails_admin.rb
+        #     model = 'SomeModel'.constantize # blocks from SomeModel get loaded
+        #     model.config model do           # blocks from initializer gets loaded
+        #       :
+        #     end
+        #
+        # Thus, sort all blocks to excute for a resource by Proc.source_path,
+        # to guarantee that blocks from 'config/initializers' evaluate before
+        # blocks defined within a model class.
+        unless @deferred_blocks.empty?
+          @deferred_blocks.
+            partition { |block| block.source_location.first =~ %r{config/initializers} }.
+            flatten.
+            each { |block| @model.instance_eval(&block) }
+          @deferred_blocks = []
+        end
+        @initialized = true
+        @model
+      end
+
+      def method_missing(method_name, *args, &block)
+        target.send(method_name, *args, &block)
+      end
+
+      def respond_to_missing?(method_name, include_private = false)
+        super || target.respond_to?(method_name, include_private)
+      end
+    end
+  end
+end

lib/rails_admin/config/model.rb

@@ -36,8 +36,10 @@ def initialize(entity)
           case entity
           when RailsAdmin::AbstractModel
             entity
-          when Class, String, Symbol
+          when Class, String
             RailsAdmin::AbstractModel.new(entity)
+          when Symbol
+            RailsAdmin::AbstractModel.new(entity.to_s)
           else
             RailsAdmin::AbstractModel.new(entity.class)
           end

lib/rails_admin/engine.rb

@@ -35,25 +35,16 @@ class Engine < Rails::Engine
       end
     end
 
-    initializer 'RailsAdmin apply configuration', after: :eager_load! do |app|
-      RailsAdmin::Config.initialize!
-
-      # Force route reload, since it doesn't reflect RailsAdmin action configuration yet
-      app.reload_routes!
-    end
-
-    initializer 'RailsAdmin precompile hook', group: :all, after: 'RailsAdmin apply configuration' do |app|
+    initializer 'RailsAdmin precompile hook', group: :all do |app|
       case RailsAdmin.config.asset_source
       when :sprockets
-        if defined?(Sprockets)
-          app.config.assets.precompile += %w[
-            rails_admin/application.js
-            rails_admin/application.css
-          ]
-          app.config.assets.paths << RailsAdmin::Engine.root.join('src')
-          require 'rails_admin/support/esmodule_preprocessor'
-          Sprockets.register_preprocessor 'application/javascript', RailsAdmin::ESModulePreprocessor
-        end
+        app.config.assets.precompile += %w[
+          rails_admin/application.js
+          rails_admin/application.css
+        ]
+        app.config.assets.paths << RailsAdmin::Engine.root.join('src')
+        require 'rails_admin/support/esmodule_preprocessor'
+        Sprockets.register_preprocessor 'application/javascript', RailsAdmin::ESModulePreprocessor
       when :importmap
         self.importmap = Importmap::Map.new.draw(app.root.join('config/importmap.rails_admin.rb'))
       end

spec/dummy_app/app/active_record/team.rb

@@ -28,4 +28,8 @@ def color_enum
   scope :green, -> { where(color: 'red') }
   scope :red, -> { where(color: 'red') }
   scope :white, -> { where(color: 'white') }
+
+  rails_admin do
+    field :color, :color
+  end
 end

spec/dummy_app/config/application.rb

@@ -36,7 +36,7 @@ class Application < Rails::Application
     # -- all .rb files in that directory are automatically loaded.
     config.load_defaults Rails.version[0, 3]
     config.eager_load_paths.reject! { |p| p =~ %r{/app/([^/]+)} && !%W[controllers jobs locales mailers #{CI_ORM}].include?(Regexp.last_match[1]) }
-    config.eager_load_paths += %W[#{config.root}/app/#{CI_ORM}/eager_loaded]
+    config.eager_load_paths += %W[#{config.root}/app/eager_loaded]
     config.autoload_paths += %W[#{config.root}/lib]
     config.i18n.load_path += Dir[Rails.root.join('app', 'locales', '*.{rb,yml}').to_s]
     config.active_record.time_zone_aware_types = %i[datetime time] if CI_ORM == :active_record

spec/dummy_app/config/initializers/rails_admin.rb

@@ -2,8 +2,8 @@
 
 RailsAdmin.config do |c|
   c.asset_source = CI_ASSET
-  c.model 'Team' do
+  c.model Team do
     include_all_fields
-    field :color, :color
+    field :color, :hidden
   end
 end