forgetting_assignment avoid serialization

`#forgetting_assignment` was introduced with 07723c2, and it
involved deserializing and then serializing current value.

This is an unnecessary extra operation, and also it
is a problem for custom attribute types where `#serialize` and
`#deserialize` are not inverse to each other operations.

This is a problem for example when using oracle-enhanced-adapter.

fixes rails#44317 and rails#42738 and rsim/oracle-enhanced#2268

activemodel/lib/active_model/attribute.rb

@@ -68,8 +68,16 @@ def changed_in_place?
       has_been_read? && type.changed_in_place?(original_value_for_database, value)
     end
 
+    # Returns an attribute that no longer remembers previous assignments.
     def forgetting_assignment
-      with_value_from_database(value_for_database)
+      case
+      when changed_in_place?
+        with_cast_value(value)
+      when changed_from_assignment?
+        dup_with_forgetting_assignment
+      else
+        self
+      end
     end
 
     def with_value_from_user(value)
@@ -155,6 +163,10 @@ def initialize_dup(other)
         end
       end
 
+      def dup_with_forgetting_assignment
+        self.dup.forget_original_assignment!
+      end
+
       def changed_from_assignment?
         assigned? && type.changed?(original_value, value, value_before_type_cast)
       end
@@ -163,6 +175,13 @@ def _original_value_for_database
         type.serialize(original_value)
       end
 
+    protected
+      # only used when duplicating attribute before ever returning to the user
+      def forget_original_assignment!
+        @original_attribute = nil
+        self
+      end
+
       class FromDatabase < Attribute # :nodoc:
         def type_cast(value)
           type.deserialize(value)
@@ -185,13 +204,22 @@ def came_from_user?
       end
 
       class WithCastValue < Attribute # :nodoc:
+        def initialize(*args, &block)
+          super
+
+          @initial_cast_value = !value_before_type_cast || value_before_type_cast.frozen? ? value_before_type_cast : value_before_type_cast.dup.freeze
+        end
+
         def type_cast(value)
           value
         end
 
         def changed_in_place?
-          false
+          initial_cast_value != value
         end
+
+        private
+          attr_reader :initial_cast_value
       end
 
       class Null < Attribute # :nodoc:

activemodel/test/cases/attribute_test.rb

@@ -227,6 +227,47 @@ def assert_valid_value(*)
 
       assert changed.changed? # Check to avoid a false positive
       assert_not_predicate forgotten, :changed?
+      assert_equal "foo", forgotten.value
+    end
+
+    class SerializingNonDeserializingType < Type::String
+      def serialize(value)
+        "serialized: #{value}"
+      end
+    end
+
+    test "forgetting assignments works when serialize/deserialize are not inverse" do
+      from_user = Attribute.from_user(:custom, "foo", SerializingNonDeserializingType.new).forgetting_assignment
+      from_db = Attribute.from_database(:custom, "foo", SerializingNonDeserializingType.new).forgetting_assignment
+      from_cast = Attribute.with_cast_value(:custom, "foo", SerializingNonDeserializingType.new).forgetting_assignment
+
+      assert_equal "foo", from_user.value
+      assert_equal "foo", from_db.value
+      assert_equal "foo", from_cast.value
+    end
+
+    test "forgetting assignments after assigning attribute" do
+      from_db = Attribute.from_database(:custom, +"bar", Type::String.new)
+      assigned = from_db.with_value_from_user("foo")
+      forgotten = assigned.forgetting_assignment
+
+      assert assigned.changed?
+      assert_not_predicate forgotten, :changed?
+      assert_equal "foo", forgotten.value
+    end
+
+    test "forgetting assignments after in-place mutation" do
+      from_user = Attribute.from_user(:custom, +"foo", Type::String.new)
+      from_db = Attribute.from_database(:custom, +"foo", Type::String.new)
+      from_cast = Attribute.with_cast_value(:custom, +"foo", Type::String.new)
+
+      from_user.value << " user"
+      from_db.value << " db"
+      from_cast.value << " cast"
+
+      assert_equal "foo user", from_user.forgetting_assignment.value
+      assert_equal "foo db", from_db.forgetting_assignment.value
+      assert_equal "foo cast", from_cast.forgetting_assignment.value
     end
 
     test "with_value_from_user validates the value" do