Fix observers with AR::Model

activerecord/lib/active_record/attribute_assignment.rb

@@ -3,6 +3,7 @@
 module ActiveRecord
   ActiveSupport.on_load(:active_record_config) do
     mattr_accessor :whitelist_attributes, instance_accessor: false
+    mattr_accessor :mass_assignment_sanitizer, instance_accessor: false
   end
 
   module AttributeAssignment

activerecord/lib/active_record/model.rb

@@ -39,6 +39,7 @@ def #{name}; ActiveRecord::Model.#{name}; end
   module Model
     extend ActiveSupport::Concern
     extend ConnectionHandling
+    extend ActiveModel::Observing::ClassMethods
 
     # This allows us to detect an ActiveRecord::Model while it's in the process of being included.
     module Tag; end

activerecord/lib/active_record/railtie.rb

@@ -110,20 +110,21 @@ class Railtie < Rails::Railtie
 
     config.after_initialize do |app|
       ActiveSupport.on_load(:active_record) do
-        ActiveRecord::Base.instantiate_observers
+        ActiveRecord::Model.instantiate_observers
 
         ActionDispatch::Reloader.to_prepare do
-          ActiveRecord::Base.instantiate_observers
+          ActiveRecord::Model.instantiate_observers
         end
       end
 
       ActiveSupport.on_load(:active_record) do
         if app.config.use_schema_cache_dump
           filename = File.join(app.config.paths["db"].first, "schema_cache.dump")
+
           if File.file?(filename)
             cache = Marshal.load File.binread filename
             if cache.version == ActiveRecord::Migrator.current_version
-              ActiveRecord::Base.connection.schema_cache = cache
+              ActiveRecord::Model.connection.schema_cache = cache
             else
               warn "schema_cache.dump is expired. Current version is #{ActiveRecord::Migrator.current_version}, but cache version is #{cache.version}."
             end

railties/test/application/configuration_test.rb

@@ -618,5 +618,26 @@ def index
       make_basic_app
       assert app.config.colorize_logging
     end
+
+    test "config.active_record.observers" do
+      add_to_config <<-RUBY
+        config.active_record.observers = :foo_observer
+      RUBY
+
+      app_file 'app/models/foo.rb', <<-RUBY
+        class Foo < ActiveRecord::Base
+        end
+      RUBY
+
+      app_file 'app/models/foo_observer.rb', <<-RUBY
+        class FooObserver < ActiveRecord::Observer
+        end
+      RUBY
+
+      require "#{app_path}/config/environment"
+
+      ActiveRecord::Base
+      assert defined?(FooObserver)
+    end
   end
 end