Make Active Record emit significantly smaller YAML

This reduces the size of a YAML encoded Active Record object by ~80%
depending on the number of columns. There were a number of wasteful
things that occurred when we encoded the objects before that have
resulted in numerous wins

- We were emitting the result of `attributes_before_type_cast` as a hack
  to work around some laziness issues
- The name of an attribute was emitted multiple times, since the
  attribute objects were in a hash keyed by the name. We now store them
  in an array instead, and reconstruct the hash using the name
- The types were included for every attribute. This would use backrefs
  if multiple objects were encoded, but really we don't need to include
  it at all unless it differs from the type at the class level. (The
  only time that will occur is if the field is the result of a custom
  select clause)
- `original_attribute:` was included over and over and over again since
  the ivar is almost always `nil`. We've added a custom implementation
  of `encode_with` on the attribute objects to ensure we don't write the
  key when the field is `nil`.

This isn't without a cost though. Since we're no longer including the
types, an object can find itself in an invalid state if the type changes
on the class after serialization. This is the same as 4.1 and earlier,
but I think it's worth noting.

I was worried that I'd introduce some new state bugs as a result of
doing this, so I've added an additional test that asserts mutation not
being lost as the result of YAML round tripping.

Fixes #25145

1 files changed, 10 insertions, 0 deletions

diff --git a/activerecord/test/cases/yaml_serialization_test.rb b/activerecord/test/cases/yaml_serialization_test.rb
index 56909a8630..d1c9a00786 100644
--- a/activerecord/test/cases/yaml_serialization_test.rb
+++ b/activerecord/test/cases/yaml_serialization_test.rb
@@ -109,6 +109,16 @@ class YamlSerializationTest < ActiveRecord::TestCase
     assert_equal("Have a nice day", topic.content)
   end
 
+  def test_yaml_encoding_keeps_mutations
+    author = Author.first
+    author.name = "Sean"
+    dumped = YAML.load(YAML.dump(author))
+
+    assert_equal "Sean", dumped.name
+    assert_equal author.name_was, dumped.name_was
+    assert_equal author.changes, dumped.changes
+  end
+
   private
 
   def yaml_fixture(file_name)