Fixed compatibility with edge rails (rails 3 master), with correspond…

…ing tests.

lib/mongo_mapper/plugins/rails.rb

@@ -9,14 +9,22 @@ module InstanceMethods
         def to_param
           id.to_s
         end
-
+
+        def to_key
+          persisted? ? [id.to_s] : nil
+        end
+
         def to_model
           self
         end
-
+        
         def new_record?
           new?
         end
+
+        def persisted?
+          !(new_record? || destroyed?)
+        end
 
         def read_attribute(name)
           self[name]

test/unit/test_document.rb

@@ -111,6 +111,13 @@ class Post
       doc.to_param.should be_instance_of(String)
     end
 
+    should "have to_key that is array representation of id" do
+      doc = @document.new(:id => Mongo::ObjectID.new)
+      doc.save
+      doc.to_key.should == [doc.id.to_s]
+      doc.to_key.should be_instance_of(Array)
+    end
+
     should "have access to logger" do
       doc = @document.new
       doc.logger.should == @document.logger
@@ -159,6 +166,19 @@ class Post
         doc.id = '1234'
         doc.new?.should be_true
       end
+
+      should "have to_key that returns nil on non-persisted records (new records)" do
+        @document.new.to_key.should be(nil)
+      end
+    end
+
+    context "destroyed?" do
+      should "have to_key that returns nil on non-persisted records (destroyed records)" do
+        doc = @document.new
+        doc.save
+        doc.destroy
+        doc.to_key.should be(nil)
+      end
     end
 
     context "clone" do