Improve the performance of `save` and friends

The biggest source of the performance regression in these methods
occurred because dirty tracking required eagerly materializing and type
casting the assigned values. In the previous commits, I've changed dirty
tracking to perform the comparisons lazily. However, all of this is moot
when calling `save`, since `changes_applied` will be called, which just
ends up eagerly materializing everything, anyway. With the new mutation
tracker, it's easy to just compare the previous two hashes in the same
lazy fashion.

We will not have aliasing issues with this setup, which is proven by the
fact that we're able to detect nested mutation.

Before:
    User.create! 2.007k (± 7.1%) i/s -     10.098k

After:
    User.create! 2.557k (± 3.5%) i/s -     12.789k

Fixes #19859

1 files changed, 2 insertions, 2 deletions

diff --git a/activemodel/lib/active_model/dirty.rb b/activemodel/lib/active_model/dirty.rb
index 0169c20e0b..0ab8df42f5 100644
--- a/activemodel/lib/active_model/dirty.rb
+++ b/activemodel/lib/active_model/dirty.rb
@@ -203,7 +203,7 @@ module ActiveModel
       # Returns +true+ if attr_name were changed before the model was saved,
       # +false+ otherwise.
       def previous_changes_include?(attr_name)
-        @previously_changed.include?(attr_name)
+        previous_changes.include?(attr_name)
       end
 
       # Removes current changes and makes them accessible through +previous_changes+.
@@ -225,7 +225,7 @@ module ActiveModel
 
       # Handles <tt>*_previous_change</tt> for +method_missing+.
       def attribute_previous_change(attr)
-        @previously_changed[attr] if attribute_previously_changed?(attr)
+        previous_changes[attr] if attribute_previously_changed?(attr)
       end
 
       # Handles <tt>*_will_change!</tt> for +method_missing+.