Separate `dup` from `deep_dup` in the attributes hash

I'm looking to move towards a tree-like structure for dirty checking
that involves an attribute holding onto the attribute that it was
created from. This means that `changed?` can be fully encapsulated on
that object. Since the objects are immutable, in `changes_applied`, we
can simply perform a shallow dup, instead of a deep one.

I'm not sure if that will actually end up in a performance boost, but
I'd like to semantically separate these concepts regardless

1 files changed, 19 insertions, 1 deletions

diff --git a/activerecord/test/cases/attribute_set_test.rb b/activerecord/test/cases/attribute_set_test.rb
index 7524243270..5a0e463a48 100644
--- a/activerecord/test/cases/attribute_set_test.rb
+++ b/activerecord/test/cases/attribute_set_test.rb
@@ -29,7 +29,7 @@ module ActiveRecord
       assert_equal :bar, attributes[:bar].name
     end
 
-    test "duping creates a new hash and dups each attribute" do
+    test "duping creates a new hash, but does not dup the attributes" do
       builder = AttributeSet::Builder.new(foo: Type::Integer.new, bar: Type::String.new)
       attributes = builder.build_from_database(foo: 1, bar: 'foo')
 
@@ -43,6 +43,24 @@ module ActiveRecord
 
       assert_equal 1, attributes[:foo].value
       assert_equal 2, duped[:foo].value
+      assert_equal 'foobar', attributes[:bar].value
+      assert_equal 'foobar', duped[:bar].value
+    end
+
+    test "deep_duping creates a new hash and dups each attribute" do
+      builder = AttributeSet::Builder.new(foo: Type::Integer.new, bar: Type::String.new)
+      attributes = builder.build_from_database(foo: 1, bar: 'foo')
+
+      # Ensure the type cast value is cached
+      attributes[:foo].value
+      attributes[:bar].value
+
+      duped = attributes.deep_dup
+      duped.write_from_database(:foo, 2)
+      duped[:bar].value << 'bar'
+
+      assert_equal 1, attributes[:foo].value
+      assert_equal 2, duped[:foo].value
       assert_equal 'foo', attributes[:bar].value
       assert_equal 'foobar', duped[:bar].value
     end