diff --git a/activemodel/lib/active_model/attribute_mutation_tracker.rb b/activemodel/lib/active_model/attribute_mutation_tracker.rb
index 6abf37bd44..d8cd48a53b 100644
--- a/activemodel/lib/active_model/attribute_mutation_tracker.rb
+++ b/activemodel/lib/active_model/attribute_mutation_tracker.rb
@@ -1,14 +1,15 @@
 # frozen_string_literal: true
 
 require "active_support/core_ext/hash/indifferent_access"
+require "active_support/core_ext/object/duplicable"
 
 module ActiveModel
   class AttributeMutationTracker # :nodoc:
     OPTION_NOT_GIVEN = Object.new
 
-    def initialize(attributes)
+    def initialize(attributes, forced_changes = Set.new)
       @attributes = attributes
-      @forced_changes = Set.new
+      @forced_changes = forced_changes
     end
 
     def changed_attribute_names
@@ -18,24 +19,22 @@ module ActiveModel
     def changed_values
       attr_names.each_with_object({}.with_indifferent_access) do |attr_name, result|
         if changed?(attr_name)
-          result[attr_name] = attributes[attr_name].original_value
+          result[attr_name] = original_value(attr_name)
         end
       end
     end
 
     def changes
       attr_names.each_with_object({}.with_indifferent_access) do |attr_name, result|
-        change = change_to_attribute(attr_name)
-        if change
+        if change = change_to_attribute(attr_name)
           result.merge!(attr_name => change)
         end
       end
     end
 
     def change_to_attribute(attr_name)
-      attr_name = attr_name.to_s
       if changed?(attr_name)
-        [attributes[attr_name].original_value, attributes.fetch_value(attr_name)]
+        [original_value(attr_name), fetch_value(attr_name)]
       end
     end
 
@@ -44,29 +43,26 @@ module ActiveModel
     end
 
     def changed?(attr_name, from: OPTION_NOT_GIVEN, to: OPTION_NOT_GIVEN)
-      attr_name = attr_name.to_s
-      forced_changes.include?(attr_name) ||
-        attributes[attr_name].changed? &&
-        (OPTION_NOT_GIVEN == from || attributes[attr_name].original_value == from) &&
-        (OPTION_NOT_GIVEN == to || attributes[attr_name].value == to)
+      attribute_changed?(attr_name) &&
+        (OPTION_NOT_GIVEN == from || original_value(attr_name) == from) &&
+        (OPTION_NOT_GIVEN == to || fetch_value(attr_name) == to)
     end
 
     def changed_in_place?(attr_name)
-      attributes[attr_name.to_s].changed_in_place?
+      attributes[attr_name].changed_in_place?
     end
 
     def forget_change(attr_name)
-      attr_name = attr_name.to_s
       attributes[attr_name] = attributes[attr_name].forgetting_assignment
       forced_changes.delete(attr_name)
     end
 
     def original_value(attr_name)
-      attributes[attr_name.to_s].original_value
+      attributes[attr_name].original_value
     end
 
     def force_change(attr_name)
-      forced_changes << attr_name.to_s
+      forced_changes << attr_name
     end
 
     private
@@ -75,45 +71,108 @@ module ActiveModel
       def attr_names
         attributes.keys
       end
+
+      def attribute_changed?(attr_name)
+        forced_changes.include?(attr_name) || !!attributes[attr_name].changed?
+      end
+
+      def fetch_value(attr_name)
+        attributes.fetch_value(attr_name)
+      end
+  end
+
+  class ForcedMutationTracker < AttributeMutationTracker # :nodoc:
+    def initialize(attributes, forced_changes = {})
+      super
+      @finalized_changes = nil
+    end
+
+    def changed_in_place?(attr_name)
+      false
+    end
+
+    def change_to_attribute(attr_name)
+      if finalized_changes&.include?(attr_name)
+        finalized_changes[attr_name].dup
+      else
+        super
+      end
+    end
+
+    def forget_change(attr_name)
+      forced_changes.delete(attr_name)
+    end
+
+    def original_value(attr_name)
+      if changed?(attr_name)
+        forced_changes[attr_name]
+      else
+        fetch_value(attr_name)
+      end
+    end
+
+    def force_change(attr_name)
+      forced_changes[attr_name] = clone_value(attr_name) unless attribute_changed?(attr_name)
+    end
+
+    def finalize_changes
+      @finalized_changes = changes
+    end
+
+    private
+      attr_reader :finalized_changes
+
+      def attr_names
+        forced_changes.keys
+      end
+
+      def attribute_changed?(attr_name)
+        forced_changes.include?(attr_name)
+      end
+
+      def fetch_value(attr_name)
+        attributes.send(:_read_attribute, attr_name)
+      end
+
+      def clone_value(attr_name)
+        value = fetch_value(attr_name)
+        value.duplicable? ? value.clone : value
+      rescue TypeError, NoMethodError
+        value
+      end
   end
 
   class NullMutationTracker # :nodoc:
     include Singleton
 
-    def changed_attribute_names(*)
+    def changed_attribute_names
       []
     end
 
-    def changed_values(*)
+    def changed_values
       {}
     end
 
-    def changes(*)
+    def changes
       {}
     end
 
     def change_to_attribute(attr_name)
     end
 
-    def any_changes?(*)
+    def any_changes?
       false
     end
 
-    def changed?(*)
+    def changed?(attr_name, **)
       false
     end
 
-    def changed_in_place?(*)
+    def changed_in_place?(attr_name)
       false
     end
 
-    def forget_change(*)
-    end
-
-    def original_value(*)
-    end
-
-    def force_change(*)
+    def original_value(attr_name)
     end
   end
 end
diff --git a/activemodel/lib/active_model/dirty.rb b/activemodel/lib/active_model/dirty.rb
index 0d9e761b1e..ab2c9d04ae 100644
--- a/activemodel/lib/active_model/dirty.rb
+++ b/activemodel/lib/active_model/dirty.rb
@@ -1,7 +1,5 @@
 # frozen_string_literal: true
 
-require "active_support/hash_with_indifferent_access"
-require "active_support/core_ext/object/duplicable"
 require "active_model/attribute_mutation_tracker"
 
 module ActiveModel
@@ -122,9 +120,6 @@ module ActiveModel
     extend ActiveSupport::Concern
     include ActiveModel::AttributeMethods
 
-    OPTION_NOT_GIVEN = Object.new # :nodoc:
-    private_constant :OPTION_NOT_GIVEN
-
     included do
       attribute_method_suffix "_changed?", "_change", "_will_change!", "_was"
       attribute_method_suffix "_previously_changed?", "_previous_change"
@@ -145,10 +140,9 @@ module ActiveModel
     # +mutations_from_database+ to +mutations_before_last_save+ respectively.
     def changes_applied
       unless defined?(@attributes)
-        @previously_changed = changes
+        mutations_from_database.finalize_changes
       end
       @mutations_before_last_save = mutations_from_database
-      @attributes_changed_by_setter = ActiveSupport::HashWithIndifferentAccess.new
       forget_attribute_assignments
       @mutations_from_database = nil
     end
@@ -159,7 +153,7 @@ 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.
@@ -168,42 +162,37 @@ 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) &&
-        (to == OPTION_NOT_GIVEN || to == _read_attribute(attr)) &&
-        (from == OPTION_NOT_GIVEN || from == changed_attributes[attr])
+    def attribute_changed?(attr_name, **options) # :nodoc:
+      mutations_from_database.changed?(attr_name.to_s, options)
     end
 
     # Handles <tt>*_was</tt> for +method_missing+.
-    def attribute_was(attr) # :nodoc:
-      attribute_changed?(attr) ? changed_attributes[attr] : _read_attribute(attr)
+    def attribute_was(attr_name) # :nodoc:
+      mutations_from_database.original_value(attr_name.to_s)
     end
 
     # Handles <tt>*_previously_changed?</tt> for +method_missing+.
-    def attribute_previously_changed?(attr) #:nodoc:
-      previous_changes_include?(attr)
+    def attribute_previously_changed?(attr_name) # :nodoc:
+      mutations_before_last_save.changed?(attr_name.to_s)
     end
 
     # Restore all previous data of the provided attributes.
-    def restore_attributes(attributes = changed)
-      attributes.each { |attr| restore_attribute! attr }
+    def restore_attributes(attr_names = changed)
+      attr_names.each { |attr_name| restore_attribute!(attr_name) }
     end
 
     # 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
@@ -216,13 +205,7 @@ 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
     end
 
     # Returns a hash of changed attributes indicating their original
@@ -232,9 +215,7 @@ 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
+      mutations_from_database.changes
     end
 
     # Returns a hash of attributes that were changed before the model was saved.
@@ -244,27 +225,23 @@ 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:
-      mutations_from_database.changed_in_place?(attr_name)
+      mutations_from_database.changed_in_place?(attr_name.to_s)
     end
 
     private
       def clear_attribute_change(attr_name)
-        mutations_from_database.forget_change(attr_name)
+        mutations_from_database.forget_change(attr_name.to_s)
       end
 
       def mutations_from_database
-        unless defined?(@mutations_from_database)
-          @mutations_from_database = nil
-        end
         @mutations_from_database ||= if defined?(@attributes)
           ActiveModel::AttributeMutationTracker.new(@attributes)
         else
-          NullMutationTracker.instance
+          ActiveModel::ForcedMutationTracker.new(self)
         end
       end
 
@@ -276,68 +253,28 @@ 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)
+      def attribute_change(attr_name)
+        mutations_from_database.change_to_attribute(attr_name.to_s)
       end
 
       # Handles <tt>*_previous_change</tt> for +method_missing+.
-      def attribute_previous_change(attr)
-        previous_changes[attr]
+      def attribute_previous_change(attr_name)
+        mutations_before_last_save.change_to_attribute(attr_name.to_s)
       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)
-        end
-        mutations_from_database.force_change(attr)
+      def attribute_will_change!(attr_name)
+        mutations_from_database.force_change(attr_name.to_s)
       end
 
       # Handles <tt>restore_*!</tt> for +method_missing+.
-      def restore_attribute!(attr)
-        if attribute_changed?(attr)
-          __send__("#{attr}=", changed_attributes[attr])
-          clear_attribute_changes([attr])
+      def restore_attribute!(attr_name)
+        attr_name = attr_name.to_s
+        if attribute_changed?(attr_name)
+          __send__("#{attr_name}=", attribute_was(attr_name))
+          clear_attribute_change(attr_name)
         end
       end
-
-      def attributes_changed_by_setter
-        @attributes_changed_by_setter ||= ActiveSupport::HashWithIndifferentAccess.new
-      end
-
-      # Force an attribute to have a particular "before" value
-      def set_attribute_was(attr, old_value)
-        attributes_changed_by_setter[attr] = old_value
-      end
   end
 end
diff --git a/activerecord/lib/active_record/attribute_methods/dirty.rb b/activerecord/lib/active_record/attribute_methods/dirty.rb
index 45e4b8adfa..920a7b2038 100644
--- a/activerecord/lib/active_record/attribute_methods/dirty.rb
+++ b/activerecord/lib/active_record/attribute_methods/dirty.rb
@@ -29,9 +29,7 @@ module ActiveRecord
       # <tt>reload</tt> the record and clears changed attributes.
       def reload(*)
         super.tap do
-          @previously_changed = ActiveSupport::HashWithIndifferentAccess.new
           @mutations_before_last_save = nil
-          @attributes_changed_by_setter = ActiveSupport::HashWithIndifferentAccess.new
           @mutations_from_database = nil
         end
       end
@@ -51,7 +49,7 @@ module ActiveRecord
       # +to+ When passed, this method will return false unless the value was
       # changed to the given value
       def saved_change_to_attribute?(attr_name, **options)
-        mutations_before_last_save.changed?(attr_name, **options)
+        mutations_before_last_save.changed?(attr_name.to_s, options)
       end
 
       # Returns the change to an attribute during the last save. If the
@@ -63,7 +61,7 @@ module ActiveRecord
       # invoked as +saved_change_to_name+ instead of
       # <tt>saved_change_to_attribute("name")</tt>.
       def saved_change_to_attribute(attr_name)
-        mutations_before_last_save.change_to_attribute(attr_name)
+        mutations_before_last_save.change_to_attribute(attr_name.to_s)
       end
 
       # Returns the original value of an attribute before the last save.
@@ -73,7 +71,7 @@ module ActiveRecord
       # invoked as +name_before_last_save+ instead of
       # <tt>attribute_before_last_save("name")</tt>.
       def attribute_before_last_save(attr_name)
-        mutations_before_last_save.original_value(attr_name)
+        mutations_before_last_save.original_value(attr_name.to_s)
       end
 
       # Did the last call to +save+ have any changes to change?
@@ -101,7 +99,7 @@ module ActiveRecord
       # +to+ When passed, this method will return false unless the value will be
       # changed to the given value
       def will_save_change_to_attribute?(attr_name, **options)
-        mutations_from_database.changed?(attr_name, **options)
+        mutations_from_database.changed?(attr_name.to_s, options)
       end
 
       # Returns the change to an attribute that will be persisted during the
@@ -115,7 +113,7 @@ module ActiveRecord
       # If the attribute will change, the result will be an array containing the
       # original value and the new value about to be saved.
       def attribute_change_to_be_saved(attr_name)
-        mutations_from_database.change_to_attribute(attr_name)
+        mutations_from_database.change_to_attribute(attr_name.to_s)
       end
 
       # Returns the value of an attribute in the database, as opposed to the
@@ -127,7 +125,7 @@ module ActiveRecord
       # saved. It can be invoked as +name_in_database+ instead of
       # <tt>attribute_in_database("name")</tt>.
       def attribute_in_database(attr_name)
-        mutations_from_database.original_value(attr_name)
+        mutations_from_database.original_value(attr_name.to_s)
       end
 
       # Will the next call to +save+ have any changes to persist?