Adding ability to get positional operator for a document

lib/mongoid/paths.rb

@@ -3,8 +3,8 @@ module Mongoid #:nodoc:
   module Paths #:nodoc:
     extend ActiveSupport::Concern
     included do
-      cattr_accessor :_path
-      delegate :_path, :to => "self.class"
+      cattr_accessor :_path, :_position
+      delegate :_path, :_position, :to => "self.class"
     end
     module InstanceMethods
       # Return the path to this +Document+ in JSON notation, used for atomic
@@ -19,6 +19,15 @@ def path
         end
       end
 
+      # Returns the positional operator of this document for modification.
+      #
+      # Example:
+      #
+      # <tt>address.position</tt>
+      def position
+        self._position ||= (path.blank? ? "" : "#{path}.$")
+      end
+
       # Return the selector for this document to be matched exactly for use
       # with MongoDB's $ operator.
       #

spec/unit/mongoid/paths_spec.rb

@@ -84,4 +84,41 @@
       end
     end
   end
+
+  describe "#position" do
+
+    context "when the document is a parent" do
+
+      it "returns an empty string" do
+        person.position.should == ""
+      end
+    end
+
+    context "when the document is embedded" do
+
+      before do
+        person.addresses << address
+      end
+
+      it "returns the path plus $" do
+        address.position.should == "addresses.$"
+      end
+    end
+
+    context "when document embedded multiple levels" do
+
+      before do
+        address.locations << location
+        person.addresses << address
+      end
+
+      it "returns the path plus $" do
+        location.position.should == "addresses.locations.$"
+      end
+
+      it "sets the matcher class instance var" do
+        Location._position.should == "addresses.locations.$"
+      end
+    end
+  end
 end