Add option for class_attribute default (#29270)

* Allow a default value to be declared for class_attribute

* Convert to using class_attribute default rather than explicit setter

* Removed instance_accessor option by mistake

* False is a valid default value

* Documentation

16 files changed, 31 insertions, 72 deletions

diff --git a/activerecord/lib/active_record/attribute_decorators.rb b/activerecord/lib/active_record/attribute_decorators.rb
index c39e9ce4c5..5bc8527745 100644
--- a/activerecord/lib/active_record/attribute_decorators.rb
+++ b/activerecord/lib/active_record/attribute_decorators.rb
@@ -3,8 +3,7 @@ module ActiveRecord
     extend ActiveSupport::Concern
 
     included do
-      class_attribute :attribute_type_decorations, instance_accessor: false # :internal:
-      self.attribute_type_decorations = TypeDecorator.new
+      class_attribute :attribute_type_decorations, instance_accessor: false, default: TypeDecorator.new # :internal:
     end
 
     module ClassMethods # :nodoc:
diff --git a/activerecord/lib/active_record/attribute_methods/dirty.rb b/activerecord/lib/active_record/attribute_methods/dirty.rb
index bd5003d63a..76987fb8f4 100644
--- a/activerecord/lib/active_record/attribute_methods/dirty.rb
+++ b/activerecord/lib/active_record/attribute_methods/dirty.rb
@@ -14,8 +14,7 @@ module ActiveRecord
           raise "You cannot include Dirty after Timestamp"
         end
 
-        class_attribute :partial_writes, instance_writer: false
-        self.partial_writes = true
+        class_attribute :partial_writes, instance_writer: false, default: true
 
         after_create { changes_internally_applied }
         after_update { changes_internally_applied }
diff --git a/activerecord/lib/active_record/attribute_methods/time_zone_conversion.rb b/activerecord/lib/active_record/attribute_methods/time_zone_conversion.rb
index 321d039ed4..4a8d231503 100644
--- a/activerecord/lib/active_record/attribute_methods/time_zone_conversion.rb
+++ b/activerecord/lib/active_record/attribute_methods/time_zone_conversion.rb
@@ -57,11 +57,8 @@ module ActiveRecord
         mattr_accessor :time_zone_aware_attributes, instance_writer: false
         self.time_zone_aware_attributes = false
 
-        class_attribute :skip_time_zone_conversion_for_attributes, instance_writer: false
-        self.skip_time_zone_conversion_for_attributes = []
-
-        class_attribute :time_zone_aware_types, instance_writer: false
-        self.time_zone_aware_types = [:datetime, :time]
+        class_attribute :skip_time_zone_conversion_for_attributes, instance_writer: false, default: []
+        class_attribute :time_zone_aware_types, instance_writer: false, default: [ :datetime, :time ]
       end
 
       module ClassMethods
diff --git a/activerecord/lib/active_record/attributes.rb b/activerecord/lib/active_record/attributes.rb
index 75f5ba3a96..475b9beec4 100644
--- a/activerecord/lib/active_record/attributes.rb
+++ b/activerecord/lib/active_record/attributes.rb
@@ -6,8 +6,7 @@ module ActiveRecord
     extend ActiveSupport::Concern
 
     included do
-      class_attribute :attributes_to_define_after_schema_loads, instance_accessor: false # :internal:
-      self.attributes_to_define_after_schema_loads = {}
+      class_attribute :attributes_to_define_after_schema_loads, instance_accessor: false, default: {} # :internal:
     end
 
     module ClassMethods
diff --git a/activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb b/activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb
index abc15f595f..9cb7c46df5 100644
--- a/activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb
+++ b/activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb
@@ -29,8 +29,7 @@ module ActiveRecord
       # to your application.rb file:
       #
       #   ActiveRecord::ConnectionAdapters::Mysql2Adapter.emulate_booleans = false
-      class_attribute :emulate_booleans
-      self.emulate_booleans = true
+      class_attribute :emulate_booleans, default: true
 
       NATIVE_DATABASE_TYPES = {
         primary_key: "bigint auto_increment PRIMARY KEY",
diff --git a/activerecord/lib/active_record/enum.rb b/activerecord/lib/active_record/enum.rb
index 496abfc5d9..12ef58a941 100644
--- a/activerecord/lib/active_record/enum.rb
+++ b/activerecord/lib/active_record/enum.rb
@@ -95,8 +95,7 @@ module ActiveRecord
 
   module Enum
     def self.extended(base) # :nodoc:
-      base.class_attribute(:defined_enums, instance_writer: false)
-      base.defined_enums = {}
+      base.class_attribute(:defined_enums, instance_writer: false, default: {})
     end
 
     def inherited(base) # :nodoc:
diff --git a/activerecord/lib/active_record/fixtures.rb b/activerecord/lib/active_record/fixtures.rb
index c19216702c..bad6542be2 100644
--- a/activerecord/lib/active_record/fixtures.rb
+++ b/activerecord/lib/active_record/fixtures.rb
@@ -878,20 +878,12 @@ module ActiveRecord
 
     included do
       class_attribute :fixture_path, instance_writer: false
-      class_attribute :fixture_table_names
-      class_attribute :fixture_class_names
-      class_attribute :use_transactional_tests
-      class_attribute :use_instantiated_fixtures # true, false, or :no_instances
-      class_attribute :pre_loaded_fixtures
-      class_attribute :config
-
-      self.fixture_table_names = []
-      self.use_instantiated_fixtures = false
-      self.pre_loaded_fixtures = false
-      self.config = ActiveRecord::Base
-
-      self.fixture_class_names = {}
-      self.use_transactional_tests = true
+      class_attribute :fixture_table_names, default: []
+      class_attribute :fixture_class_names, default: {}
+      class_attribute :use_transactional_tests, default: true
+      class_attribute :use_instantiated_fixtures, default: false # true, false, or :no_instances
+      class_attribute :pre_loaded_fixtures, default: false
+      class_attribute :config, default: ActiveRecord::Base
     end
 
     module ClassMethods
diff --git a/activerecord/lib/active_record/inheritance.rb b/activerecord/lib/active_record/inheritance.rb
index 236a65eba7..5776807507 100644
--- a/activerecord/lib/active_record/inheritance.rb
+++ b/activerecord/lib/active_record/inheritance.rb
@@ -38,8 +38,7 @@ module ActiveRecord
     included do
       # Determines whether to store the full constant name including namespace when using STI.
       # This is true, by default.
-      class_attribute :store_full_sti_class, instance_writer: false
-      self.store_full_sti_class = true
+      class_attribute :store_full_sti_class, instance_writer: false, default: true
     end
 
     module ClassMethods
diff --git a/activerecord/lib/active_record/integration.rb b/activerecord/lib/active_record/integration.rb
index 32362fa86f..cf954852bc 100644
--- a/activerecord/lib/active_record/integration.rb
+++ b/activerecord/lib/active_record/integration.rb
@@ -11,8 +11,7 @@ module ActiveRecord
       # versioning is off. Accepts any of the symbols in <tt>Time::DATE_FORMATS</tt>.
       #
       # This is +:usec+, by default.
-      class_attribute :cache_timestamp_format, instance_writer: false
-      self.cache_timestamp_format = :usec
+      class_attribute :cache_timestamp_format, instance_writer: false, default: :usec
 
       ##
       # :singleton-method:
@@ -20,8 +19,7 @@ module ActiveRecord
       # by a changing version in the #cache_version method.
       #
       # This is +false+, by default until Rails 6.0.
-      class_attribute :cache_versioning, instance_writer: false
-      self.cache_versioning = false
+      class_attribute :cache_versioning, instance_writer: false, default: false
     end
 
     # Returns a +String+, which Action Pack uses for constructing a URL to this
diff --git a/activerecord/lib/active_record/locking/optimistic.rb b/activerecord/lib/active_record/locking/optimistic.rb
index 78ce9f8291..3c7110369b 100644
--- a/activerecord/lib/active_record/locking/optimistic.rb
+++ b/activerecord/lib/active_record/locking/optimistic.rb
@@ -51,8 +51,7 @@ module ActiveRecord
       extend ActiveSupport::Concern
 
       included do
-        class_attribute :lock_optimistically, instance_writer: false
-        self.lock_optimistically = true
+        class_attribute :lock_optimistically, instance_writer: false, default: true
       end
 
       def locking_enabled? #:nodoc:
diff --git a/activerecord/lib/active_record/model_schema.rb b/activerecord/lib/active_record/model_schema.rb
index 013562708c..1179a60e9b 100644
--- a/activerecord/lib/active_record/model_schema.rb
+++ b/activerecord/lib/active_record/model_schema.rb
@@ -130,26 +130,13 @@ module ActiveRecord
     included do
       mattr_accessor :primary_key_prefix_type, instance_writer: false
 
-      class_attribute :table_name_prefix, instance_writer: false
-      self.table_name_prefix = ""
-
-      class_attribute :table_name_suffix, instance_writer: false
-      self.table_name_suffix = ""
-
-      class_attribute :schema_migrations_table_name, instance_accessor: false
-      self.schema_migrations_table_name = "schema_migrations"
-
-      class_attribute :internal_metadata_table_name, instance_accessor: false
-      self.internal_metadata_table_name = "ar_internal_metadata"
-
-      class_attribute :protected_environments, instance_accessor: false
-      self.protected_environments = ["production"]
-
-      class_attribute :pluralize_table_names, instance_writer: false
-      self.pluralize_table_names = true
-
-      class_attribute :ignored_columns, instance_accessor: false
-      self.ignored_columns = [].freeze
+      class_attribute :table_name_prefix, instance_writer: false, default: ""
+      class_attribute :table_name_suffix, instance_writer: false, default: ""
+      class_attribute :schema_migrations_table_name, instance_accessor: false, default: "schema_migrations"
+      class_attribute :internal_metadata_table_name, instance_accessor: false, default: "ar_internal_metadata"
+      class_attribute :protected_environments, instance_accessor: false, default: [ "production" ]
+      class_attribute :pluralize_table_names, instance_writer: false, default: true
+      class_attribute :ignored_columns, instance_accessor: false, default: [].freeze
 
       self.inheritance_column = "type"
 
diff --git a/activerecord/lib/active_record/nested_attributes.rb b/activerecord/lib/active_record/nested_attributes.rb
index 3f39fb84e8..917bc76993 100644
--- a/activerecord/lib/active_record/nested_attributes.rb
+++ b/activerecord/lib/active_record/nested_attributes.rb
@@ -10,8 +10,7 @@ module ActiveRecord
     extend ActiveSupport::Concern
 
     included do
-      class_attribute :nested_attributes_options, instance_writer: false
-      self.nested_attributes_options = {}
+      class_attribute :nested_attributes_options, instance_writer: false, default: {}
     end
 
     # = Active Record Nested Attributes
diff --git a/activerecord/lib/active_record/readonly_attributes.rb b/activerecord/lib/active_record/readonly_attributes.rb
index 6274996ab8..af6473d250 100644
--- a/activerecord/lib/active_record/readonly_attributes.rb
+++ b/activerecord/lib/active_record/readonly_attributes.rb
@@ -3,8 +3,7 @@ module ActiveRecord
     extend ActiveSupport::Concern
 
     included do
-      class_attribute :_attr_readonly, instance_accessor: false
-      self._attr_readonly = []
+      class_attribute :_attr_readonly, instance_accessor: false, default: []
     end
 
     module ClassMethods
diff --git a/activerecord/lib/active_record/reflection.rb b/activerecord/lib/active_record/reflection.rb
index 65fdbc2fe4..d6154c9a38 100644
--- a/activerecord/lib/active_record/reflection.rb
+++ b/activerecord/lib/active_record/reflection.rb
@@ -8,10 +8,8 @@ module ActiveRecord
     extend ActiveSupport::Concern
 
     included do
-      class_attribute :_reflections, instance_writer: false
-      class_attribute :aggregate_reflections, instance_writer: false
-      self._reflections = {}
-      self.aggregate_reflections = {}
+      class_attribute :_reflections, instance_writer: false, default: {}
+      class_attribute :aggregate_reflections, instance_writer: false, default: {}
     end
 
     def self.create(macro, name, scope, options, ar)
diff --git a/activerecord/lib/active_record/scoping/default.rb b/activerecord/lib/active_record/scoping/default.rb
index 2daa48859a..ba5cc29ac1 100644
--- a/activerecord/lib/active_record/scoping/default.rb
+++ b/activerecord/lib/active_record/scoping/default.rb
@@ -5,11 +5,8 @@ module ActiveRecord
 
       included do
         # Stores the default scope for the class.
-        class_attribute :default_scopes, instance_writer: false, instance_predicate: false
-        class_attribute :default_scope_override, instance_writer: false, instance_predicate: false
-
-        self.default_scopes = []
-        self.default_scope_override = nil
+        class_attribute :default_scopes, instance_writer: false, instance_predicate: false, default: []
+        class_attribute :default_scope_override, instance_writer: false, instance_predicate: false, default: nil
       end
 
       module ClassMethods
diff --git a/activerecord/lib/active_record/timestamp.rb b/activerecord/lib/active_record/timestamp.rb
index 09d8d1cdd4..55f3a194a9 100644
--- a/activerecord/lib/active_record/timestamp.rb
+++ b/activerecord/lib/active_record/timestamp.rb
@@ -43,8 +43,7 @@ module ActiveRecord
     extend ActiveSupport::Concern
 
     included do
-      class_attribute :record_timestamps
-      self.record_timestamps = true
+      class_attribute :record_timestamps, default: true
     end
 
     def initialize_dup(other) # :nodoc: