Stop creating ApplicationRecord on model generation

When generating models, we created ApplicationRecord in the default
location if no file existed there.  That was annoying for people who
moved it to somewhere else in the autoload path.  At this point, the
vast majority of apps should have either run the upgrade script or
generated a model since upgrading.  For those that haven't the error
message after generating a new model should be helpful:

   NameError: uninitialized constant ApplicationRecord

To ease friction in that case, this also adds a generator for
ApplicationRecord.

4 files changed, 31 insertions, 23 deletions

diff --git a/activerecord/CHANGELOG.md b/activerecord/CHANGELOG.md
index afc459ef68..a656c767c0 100644
--- a/activerecord/CHANGELOG.md
+++ b/activerecord/CHANGELOG.md
@@ -1,3 +1,8 @@
+*   ApplicationRecord is no longer generated when generating models.  If you
+    need to generate it, it can be created with `rails g application_record`.
+
+    *Lisa Ugray*
+
 *   Fix `COUNT(DISTINCT ...)` with `ORDER BY` and `LIMIT` to keep the existing select list.
 
     *Ryuta Kamizono*
diff --git a/activerecord/lib/rails/generators/active_record/application_record/application_record_generator.rb b/activerecord/lib/rails/generators/active_record/application_record/application_record_generator.rb
new file mode 100644
index 0000000000..d18330f5b2
--- /dev/null
+++ b/activerecord/lib/rails/generators/active_record/application_record/application_record_generator.rb
@@ -0,0 +1,26 @@
+# frozen_string_literal: true
+
+require "rails/generators/active_record"
+
+module ActiveRecord
+  module Generators # :nodoc:
+    class ApplicationRecordGenerator < ::Rails::Generators::Base # :nodoc:
+      source_root File.expand_path("templates", __dir__)
+
+      # FIXME: Change this file to a symlink once RubyGems 2.5.0 is required.
+      def create_application_record
+        template "application_record.rb", application_record_file_name
+      end
+
+      private
+
+        def application_record_file_name
+          @application_record_file_name ||= if namespaced?
+                                              "app/models/#{namespaced_path}/application_record.rb"
+                                            else
+                                              "app/models/application_record.rb"
+                                            end
+        end
+    end
+  end
+end
diff --git a/activerecord/lib/rails/generators/active_record/model/templates/application_record.rb b/activerecord/lib/rails/generators/active_record/application_record/templates/application_record.rb
index 60050e0bf8..60050e0bf8 100644
--- a/activerecord/lib/rails/generators/active_record/model/templates/application_record.rb
+++ b/activerecord/lib/rails/generators/active_record/application_record/templates/application_record.rb
diff --git a/activerecord/lib/rails/generators/active_record/model/model_generator.rb b/activerecord/lib/rails/generators/active_record/model/model_generator.rb
index 7a1bb7efdc..25e54f3ac8 100644
--- a/activerecord/lib/rails/generators/active_record/model/model_generator.rb
+++ b/activerecord/lib/rails/generators/active_record/model/model_generator.rb
@@ -23,13 +23,11 @@ module ActiveRecord
       end
 
       def create_model_file
-        generate_application_record
         template "model.rb", File.join("app/models", class_path, "#{file_name}.rb")
       end
 
       def create_module_file
         return if regular_class_path.empty?
-        generate_application_record
         template "module.rb", File.join("app/models", "#{class_path.join('/')}.rb") if behavior == :invoke
       end
 
@@ -41,31 +39,10 @@ module ActiveRecord
           attributes.select { |a| !a.reference? && a.has_index? }
         end
 
-        # FIXME: Change this file to a symlink once RubyGems 2.5.0 is required.
-        def generate_application_record
-          if behavior == :invoke && !application_record_exist?
-            template "application_record.rb", application_record_file_name
-          end
-        end
-
         # Used by the migration template to determine the parent name of the model
         def parent_class_name
           options[:parent] || "ApplicationRecord"
         end
-
-        def application_record_exist?
-          file_exist = nil
-          in_root { file_exist = File.exist?(application_record_file_name) }
-          file_exist
-        end
-
-        def application_record_file_name
-          @application_record_file_name ||= if mountable_engine?
-            "app/models/#{namespaced_path}/application_record.rb"
-          else
-            "app/models/application_record.rb"
-          end
-        end
     end
   end
 end