Don't calculate in-place changes on attribute assignment

When an attribute is assigned, we determine if it was already marked as
changed so we can determine if we need to clear the changes, or mark it
as changed. Since this only affects the `attributes_changed_by_setter`
hash, in-place changes are irrelevant to this process. Since calculating
in-place changes can be expensive, we can just skip it here.

I also added a test for the only edge case I could think of that would
be affected by this change.

3 files changed, 10 insertions, 1 deletions

diff --git a/activemodel/lib/active_model/dirty.rb b/activemodel/lib/active_model/dirty.rb
index afba9bab0d..614bc6a5d8 100644
--- a/activemodel/lib/active_model/dirty.rb
+++ b/activemodel/lib/active_model/dirty.rb
@@ -189,6 +189,7 @@ module ActiveModel
       def changes_include?(attr_name)
         attributes_changed_by_setter.include?(attr_name)
       end
+      alias attribute_changed_by_setter? changes_include?
 
       # Removes current changes and makes them accessible through +previous_changes+.
       def changes_applied # :doc:
diff --git a/activerecord/lib/active_record/attribute_methods/dirty.rb b/activerecord/lib/active_record/attribute_methods/dirty.rb
index ce7f575150..bce9c5e1e3 100644
--- a/activerecord/lib/active_record/attribute_methods/dirty.rb
+++ b/activerecord/lib/active_record/attribute_methods/dirty.rb
@@ -108,7 +108,7 @@ module ActiveRecord
       end
 
       def save_changed_attribute(attr, old_value)
-        if attribute_changed?(attr)
+        if attribute_changed_by_setter?(attr)
           clear_attribute_changes(attr) unless _field_changed?(attr, old_value)
         else
           set_attribute_was(attr, old_value) if _field_changed?(attr, old_value)
diff --git a/activerecord/test/cases/dirty_test.rb b/activerecord/test/cases/dirty_test.rb
index ae4a8aab2c..192ba6f7cd 100644
--- a/activerecord/test/cases/dirty_test.rb
+++ b/activerecord/test/cases/dirty_test.rb
@@ -721,6 +721,14 @@ class DirtyTest < ActiveRecord::TestCase
     assert record.save
   end
 
+  test "mutating and then assigning doesn't remove the change" do
+    pirate = Pirate.create!(catchphrase: "arrrr")
+    pirate.catchphrase << " matey!"
+    pirate.catchphrase = "arrrr matey!"
+
+    assert pirate.catchphrase_changed?(from: "arrrr", to: "arrrr matey!")
+  end
+
   private
     def with_partial_writes(klass, on = true)
       old = klass.partial_writes?