PERF: Recover `changes_applied` performance (#31698)

#30985 caused `object.save` performance regression since calling
`changes` in `changes_applied` is very slow.
We don't need to call the expensive method in `changes_applied` as long
as `@attributes` is tracked by mutation tracker.

https://gist.github.com/kamipo/1a9f4f3891803b914fc72ede98268aa2

Before:

```
Warming up --------------------------------------
create_string_columns
                        73.000  i/100ms
Calculating -------------------------------------
create_string_columns
                        722.256  (± 5.8%) i/s -      3.650k in   5.073031s
```

After:

```
Warming up --------------------------------------
create_string_columns
                        96.000  i/100ms
Calculating -------------------------------------
create_string_columns
                        950.224  (± 7.7%) i/s -      4.800k in   5.084837s
```

2 files changed, 43 insertions, 70 deletions

diff --git a/activemodel/lib/active_model/attribute_mutation_tracker.rb b/activemodel/lib/active_model/attribute_mutation_tracker.rb
index c67e1b809a..f55613ecd5 100644
--- a/activemodel/lib/active_model/attribute_mutation_tracker.rb
+++ b/activemodel/lib/active_model/attribute_mutation_tracker.rb
@@ -35,6 +35,10 @@ module ActiveModel
       end
     end
 
+    def changed_attribute_names
+      attr_names.select { |attr| changed?(attr) }
+    end
+
     def any_changes?
       attr_names.any? { |attr| changed?(attr) }
     end
@@ -109,8 +113,5 @@ module ActiveModel
 
     def original_value(*)
     end
-
-    def force_change(*)
-    end
   end
 end
diff --git a/activemodel/lib/active_model/dirty.rb b/activemodel/lib/active_model/dirty.rb
index d2ebd18107..0044fde6c5 100644
--- a/activemodel/lib/active_model/dirty.rb
+++ b/activemodel/lib/active_model/dirty.rb
@@ -3,6 +3,7 @@
 require "active_support/hash_with_indifferent_access"
 require "active_support/core_ext/object/duplicable"
 require "active_model/attribute_mutation_tracker"
+require "active_model/attribute_set"
 
 module ActiveModel
   # == Active \Model \Dirty
@@ -142,9 +143,8 @@ module ActiveModel
     end
 
     def changes_applied # :nodoc:
-      @previously_changed = changes
+      _prepare_changes
       @mutations_before_last_save = mutations_from_database
-      @attributes_changed_by_setter = ActiveSupport::HashWithIndifferentAccess.new
       forget_attribute_assignments
       @mutations_from_database = nil
     end
@@ -155,7 +155,7 @@ module ActiveModel
     #   person.name = 'bob'
     #   person.changed? # => true
     def changed?
-      changed_attributes.present?
+      mutations_from_database.any_changes?
     end
 
     # Returns an array with the name of the attributes with unsaved changes.
@@ -164,24 +164,24 @@ module ActiveModel
     #   person.name = 'bob'
     #   person.changed # => ["name"]
     def changed
-      changed_attributes.keys
+      mutations_from_database.changed_attribute_names
     end
 
     # Handles <tt>*_changed?</tt> for +method_missing+.
     def attribute_changed?(attr, from: OPTION_NOT_GIVEN, to: OPTION_NOT_GIVEN) # :nodoc:
-      !!changes_include?(attr) &&
+      !!mutations_from_database.changed?(attr) &&
         (to == OPTION_NOT_GIVEN || to == _read_attribute(attr)) &&
-        (from == OPTION_NOT_GIVEN || from == changed_attributes[attr])
+        (from == OPTION_NOT_GIVEN || from == attribute_was(attr))
     end
 
     # Handles <tt>*_was</tt> for +method_missing+.
     def attribute_was(attr) # :nodoc:
-      attribute_changed?(attr) ? changed_attributes[attr] : _read_attribute(attr)
+      mutations_from_database.original_value(attr)
     end
 
     # Handles <tt>*_previously_changed?</tt> for +method_missing+.
     def attribute_previously_changed?(attr) #:nodoc:
-      previous_changes_include?(attr)
+      mutations_before_last_save.changed?(attr)
     end
 
     # Restore all previous data of the provided attributes.
@@ -191,15 +191,12 @@ module ActiveModel
 
     # Clears all dirty data: current changes and previous changes.
     def clear_changes_information
-      @previously_changed = ActiveSupport::HashWithIndifferentAccess.new
       @mutations_before_last_save = nil
-      @attributes_changed_by_setter = ActiveSupport::HashWithIndifferentAccess.new
       forget_attribute_assignments
       @mutations_from_database = nil
     end
 
     def clear_attribute_changes(attr_names)
-      attributes_changed_by_setter.except!(*attr_names)
       attr_names.each do |attr_name|
         clear_attribute_change(attr_name)
       end
@@ -212,13 +209,7 @@ module ActiveModel
     #   person.name = 'robert'
     #   person.changed_attributes # => {"name" => "bob"}
     def changed_attributes
-      # This should only be set by methods which will call changed_attributes
-      # multiple times when it is known that the computed value cannot change.
-      if defined?(@cached_changed_attributes)
-        @cached_changed_attributes
-      else
-        attributes_changed_by_setter.reverse_merge(mutations_from_database.changed_values).freeze
-      end
+      mutations_from_database.changed_values.freeze
     end
 
     # Returns a hash of changed attributes indicating their original
@@ -228,9 +219,8 @@ module ActiveModel
     #   person.name = 'bob'
     #   person.changes # => { "name" => ["bill", "bob"] }
     def changes
-      cache_changed_attributes do
-        ActiveSupport::HashWithIndifferentAccess[changed.map { |attr| [attr, attribute_change(attr)] }]
-      end
+      _prepare_changes
+      mutations_from_database.changes
     end
 
     # Returns a hash of attributes that were changed before the model was saved.
@@ -240,8 +230,7 @@ module ActiveModel
     #   person.save
     #   person.previous_changes # => {"name" => ["bob", "robert"]}
     def previous_changes
-      @previously_changed ||= ActiveSupport::HashWithIndifferentAccess.new
-      @previously_changed.merge(mutations_before_last_save.changes)
+      mutations_before_last_save.changes
     end
 
     def attribute_changed_in_place?(attr_name) # :nodoc:
@@ -257,11 +246,17 @@ module ActiveModel
         unless defined?(@mutations_from_database)
           @mutations_from_database = nil
         end
-        @mutations_from_database ||= if defined?(@attributes)
-          ActiveModel::AttributeMutationTracker.new(@attributes)
-        else
-          NullMutationTracker.instance
+
+        unless defined?(@attributes)
+          @_pseudo_attributes = true
+          @attributes = AttributeSet.new(
+            Hash.new { |h, attr|
+              h[attr] = Attribute.with_cast_value(attr, _clone_attribute(attr), Type.default_value)
+            }
+          )
         end
+
+        @mutations_from_database ||= ActiveModel::AttributeMutationTracker.new(@attributes)
       end
 
       def forget_attribute_assignments
@@ -272,68 +267,45 @@ module ActiveModel
         @mutations_before_last_save ||= ActiveModel::NullMutationTracker.instance
       end
 
-      def cache_changed_attributes
-        @cached_changed_attributes = changed_attributes
-        yield
-      ensure
-        clear_changed_attributes_cache
-      end
-
-      def clear_changed_attributes_cache
-        remove_instance_variable(:@cached_changed_attributes) if defined?(@cached_changed_attributes)
-      end
-
-      # Returns +true+ if attr_name is changed, +false+ otherwise.
-      def changes_include?(attr_name)
-        attributes_changed_by_setter.include?(attr_name) || mutations_from_database.changed?(attr_name)
-      end
-      alias attribute_changed_by_setter? changes_include?
-
-      # Returns +true+ if attr_name were changed before the model was saved,
-      # +false+ otherwise.
-      def previous_changes_include?(attr_name)
-        previous_changes.include?(attr_name)
-      end
-
       # Handles <tt>*_change</tt> for +method_missing+.
       def attribute_change(attr)
-        [changed_attributes[attr], _read_attribute(attr)] if attribute_changed?(attr)
+        [attribute_was(attr), _read_attribute(attr)] if attribute_changed?(attr)
       end
 
       # Handles <tt>*_previous_change</tt> for +method_missing+.
       def attribute_previous_change(attr)
-        previous_changes[attr] if attribute_previously_changed?(attr)
+        mutations_before_last_save.change_to_attribute(attr)
       end
 
       # Handles <tt>*_will_change!</tt> for +method_missing+.
       def attribute_will_change!(attr)
-        unless attribute_changed?(attr)
-          begin
-            value = _read_attribute(attr)
-            value = value.duplicable? ? value.clone : value
-          rescue TypeError, NoMethodError
-          end
-
-          set_attribute_was(attr, value)
+        attr = attr.to_s
+        mutations_from_database.force_change(attr).tap do
+          @attributes[attr] if defined?(@_pseudo_attributes)
         end
-        mutations_from_database.force_change(attr)
       end
 
       # Handles <tt>restore_*!</tt> for +method_missing+.
       def restore_attribute!(attr)
         if attribute_changed?(attr)
-          __send__("#{attr}=", changed_attributes[attr])
+          __send__("#{attr}=", attribute_was(attr))
           clear_attribute_changes([attr])
         end
       end
 
-      def attributes_changed_by_setter
-        @attributes_changed_by_setter ||= ActiveSupport::HashWithIndifferentAccess.new
+      def _prepare_changes
+        if defined?(@_pseudo_attributes)
+          changed.each do |attr|
+            @attributes.write_from_user(attr, _read_attribute(attr))
+          end
+        end
       end
 
-      # Force an attribute to have a particular "before" value
-      def set_attribute_was(attr, old_value)
-        attributes_changed_by_setter[attr] = old_value
+      def _clone_attribute(attr)
+        value = _read_attribute(attr)
+        value.duplicable? ? value.clone : value
+      rescue TypeError, NoMethodError
+        value
       end
   end
 end