Don't crash when mutating attributes in a getter

If a getter has side effects on the DB, `changes_applied` will be called
twice. The second time will try and remove the changed attributes cache,
and will crash because it's already been unset. This also demonstrates
that we shouldn't assume that calling getters won't change the value of
`changed_attributes`, and we need to clear the cache if an attribute is
modified.

Fixes #20531.

1 files changed, 16 insertions, 0 deletions

diff --git a/activerecord/test/cases/dirty_test.rb b/activerecord/test/cases/dirty_test.rb
index 216f228142..f5aaf22e13 100644
--- a/activerecord/test/cases/dirty_test.rb
+++ b/activerecord/test/cases/dirty_test.rb
@@ -703,6 +703,22 @@ class DirtyTest < ActiveRecord::TestCase
     assert pirate.catchphrase_changed?(from: "arrrr", to: "arrrr matey!")
   end
 
+  test "getters with side effects are allowed" do
+    klass = Class.new(Pirate) do
+      def catchphrase
+        if super.blank?
+          update_attribute(:catchphrase, "arr") # what could possibly go wrong?
+        end
+        super
+      end
+    end
+
+    pirate = klass.create!(catchphrase: "lol")
+    pirate.update_attribute(:catchphrase, nil)
+
+    assert_equal "arr", pirate.catchphrase
+  end
+
   private
     def with_partial_writes(klass, on = true)
       old = klass.partial_writes?