- Finishing off serialization refactor.

- Any type can now be localized. Closes #1558.
- Serialization now only needs a #mongoize instance method and
  .demongoize class method on any object to be able to be a custom type.
  Closes #1604.

lib/mongoid/extensions/float.rb

@@ -21,7 +21,11 @@ module ClassMethods
         #
         # @since 3.0.0
         def mongoize(object)
-          object ? (__numeric__(object) rescue 0.0) : object
+          unless object.blank?
+            __numeric__(object) rescue 0.0
+          else
+            nil
+          end
         end
       end
     end

lib/mongoid/extensions/integer.rb

@@ -31,7 +31,11 @@ module ClassMethods
         #
         # @since 3.0.0
         def mongoize(object)
-          object ? (__numeric__(object).to_i rescue 0) : object
+          unless object.blank?
+            __numeric__(object).to_i rescue 0
+          else
+            nil
+          end
         end
       end
     end

lib/mongoid/fields/foreign_key.rb

@@ -3,6 +3,42 @@ module Mongoid
   module Fields
     class ForeignKey < Standard
 
+      # Adds the atomic changes for this type of resizable field.
+      #
+      # @example Add the atomic changes.
+      #   field.add_atomic_changes(doc, "key", {}, [], [])
+      #
+      # @todo: Durran: Refactor, big time.
+      #
+      # @param [ Document ] document The document to add to.
+      # @param [ String ] name The name of the field.
+      # @param [ String ] key The atomic location of the field.
+      # @param [ Hash ] mods The current modifications.
+      # @param [ Array ] new The new elements to add.
+      # @param [ Array ] old The old elements getting removed.
+      #
+      # @since 2.4.0
+      def add_atomic_changes(document, name, key, mods, new_elements, old_elements)
+        old = (old_elements || [])
+        new = (new_elements || [])
+        if new.length > old.length
+          if new.first(old.length) == old
+            document.atomic_array_add_to_sets[key] = new.drop(old.length)
+          else
+            mods[key] = document.attributes[name]
+          end
+        elsif new.length < old.length
+          pulls = old - new
+          if new == old - pulls
+            document.atomic_array_pulls[key] = pulls
+          else
+            mods[key] = document.attributes[name]
+          end
+        elsif new != old
+          mods[key] = document.attributes[name]
+        end
+      end
+
       # Is this field a foreign key?
       #
       # @example Is the field a foreign key?
@@ -15,6 +51,10 @@ def foreign_key?
         true
       end
 
+      def evolve(object)
+        object.is_a?(Document) ? object.id : mongoize(object)
+      end
+
       def mongoize(object)
         type.__mongoize_fk__(constraint, object, object_id_field?)
       end

spec/mongoid/extensions/float_spec.rb

@@ -91,8 +91,8 @@
 
       context "when the string is empty" do
 
-        it "returns 0" do
-          Float.mongoize("").should eq(0)
+        it "returns nil" do
+          Float.mongoize("").should be_nil
         end
       end
 

spec/mongoid/extensions/integer_spec.rb

@@ -91,8 +91,8 @@
 
       context "when the string is empty" do
 
-        it "returns 0" do
-          Integer.mongoize("").should eq(0)
+        it "returns nil" do
+          Integer.mongoize("").should be_nil
         end
       end