Model#becomes properly keeps the same id. [ fix #2504 ]

CHANGELOG.md

@@ -7,6 +7,8 @@ For instructions on upgrading to newer versions, visit
 
 ### Resolved Issues
 
+* \#2504 `Model#becomes` properly keeps the same id.
+
 * \#2494 All explicit atomic operations now properly respect aliased fields.
 
 * \#2491 Removed unnecessary merge call in cascadable children.

lib/mongoid/document.rb

@@ -218,6 +218,7 @@ def becomes(klass)
         raise ArgumentError, "A class which includes Mongoid::Document is expected"
       end
       became = klass.new(as_document.__deep_copy__)
+      became.id = id
       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

@@ -885,6 +885,10 @@ class Manager < Person
           became.title.should eq('Sir')
         end
 
+        it "keeps the same object id" do
+          became.id.should eq(obj.id)
+        end
+
         context "when the document has embedded documents" do
 
           let!(:address) do