Fields need to be flagged as attribute methods:

- In order for dynamic dirty tracking methods to work, each field much
  be registered with Active Model for the method missing matchers. This
  was previously only working if the field had a value set.

- This will need a rework of how it is implemented once the next release
  of Active Model comes out. This is because until the next release all
  fields have to be marked at one time, which causes Mongoid to undef
  the entire list and reset the list with every field definition. AM
  master currently allows attributes to be added one at a time.

- Fixes #1036.

lib/mongoid/fields.rb

@@ -190,6 +190,13 @@ def add_field(name, options = {})
           fields[name] = field
           create_accessors(name, meth, options)
           process_options(field)
+
+          # @todo Durran: Refactor this once we can depend on at least
+          #   ActiveModel greater than 3.0.9. They finally have the ability then
+          #   to add attribute methods one at a time. This code will make class
+          #   load times extremely slow.
+          undefine_attribute_methods
+          define_attribute_methods(fields.keys)
         end
       end
 

spec/unit/mongoid/dirty_spec.rb

@@ -2,6 +2,25 @@
 
 describe Mongoid::Dirty do
 
+  context "when fields have been defined pre-dirty inclusion" do
+
+    let(:person) do
+      Person.new
+    end
+
+    it "defines a _change method" do
+      person.updated_at_change.should be_nil
+    end
+
+    it "defines a _changed? method" do
+      person.updated_at_changed?.should eq(false)
+    end
+
+    it "defines a _changes method" do
+      person.updated_at_was.should be_nil
+    end
+  end
+
   describe "#attribute_change" do
 
     context "when the attribute has changed" do