Attempt to fix dowcasting when the document is persisted.

lib/mongoid/document.rb

@@ -217,8 +217,14 @@ def becomes(klass)
       unless klass.include?(Mongoid::Document)
         raise ArgumentError, "A class which includes Mongoid::Document is expected"
       end
-      became = klass.new(as_document.__deep_copy__)
-      became.id = id
+
+      if changed?
+        became = klass.new(as_document.__deep_copy__)
+        became.id = id
+      else
+        became = klass.instantiate(as_document.__deep_copy__)
+      end
+
       became.instance_variable_set(:@changed_attributes, changed_attributes)
       became.instance_variable_set(:@errors, errors)
       became.instance_variable_set(:@new_record, new_record?)

spec/mongoid/document_spec.rb

@@ -997,6 +997,33 @@ class Manager < Person
         obj.level.should eq(1)
       end
     end
+
+    context "downcasting when the document is persisted" do
+
+      let(:obj) do
+        Person.create(title: 'Sir')
+      end
+
+      let(:became) do
+        obj.becomes(Manager)
+      end
+
+      context "when downcasted document is saved" do
+        before { became.save }
+
+        it "keeps the type" do
+          became.should be_an_instance_of(Manager)
+        end
+
+        it "can by queried by the parent class" do
+          Person.find(became.id).should be_an_instance_of(Manager)
+        end
+
+        it "can by queried by the main class" do
+          Manager.find(became.id).should be_an_instance_of(Manager)
+        end
+      end
+    end
   end
 
   context "when marshalling the document" do