Added a few details to SCI.

Allows getting the sci parent and root. Needed for scoped identity map
that is shared for sci models.

lib/mongo_mapper/plugins/sci.rb

@@ -13,9 +13,30 @@ def inherited(subclass)
           key :_type, String unless key?(:_type)
           subclass.instance_variable_set("@single_collection_inherited", true)
           subclass.set_collection_name(collection_name) unless subclass.embeddable?
+          subclass.single_collection_parent = self
           super
         end
 
+        def single_collection_root
+          parent = single_collection_parent || self
+          root = parent
+
+          while parent
+            parent = parent.single_collection_parent
+            root = parent unless parent.nil?
+          end
+
+          root
+        end
+
+        def single_collection_parent
+          @single_collection_parent
+        end
+
+        def single_collection_parent=(parent)
+          @single_collection_parent = parent
+        end
+
         def single_collection_inherited?
           @single_collection_inherited == true
         end

test/functional/test_sci.rb

@@ -36,6 +36,22 @@ class ::DocGrandGrandSon < ::DocGrandSon; end
       DocDaughter.collection.name.should == DocParent.collection.name
     end
 
+    should "know single_collection_parent" do
+      DocParent.single_collection_parent.should be_nil
+      DocDaughter.single_collection_parent.should      == DocParent
+      DocSon.single_collection_parent.should           == DocParent
+      DocGrandSon.single_collection_parent.should      == DocSon
+      DocGrandGrandSon.single_collection_parent.should == DocGrandSon
+    end
+
+    should "know single_collection_root" do
+      DocParent.single_collection_root.should        == DocParent
+      DocDaughter.single_collection_root.should      == DocParent
+      DocSon.single_collection_root.should           == DocParent
+      DocGrandSon.single_collection_root.should      == DocParent
+      DocGrandGrandSon.single_collection_root.should == DocParent
+    end
+
     context ".single_collection_inherited?" do
       should "be false if has not inherited" do
         DocParent.should_not be_single_collection_inherited