Re-parse options at end of class definition.

lib/mongoid/history/trackable.rb

@@ -3,8 +3,26 @@ module History
     module Trackable
       extend ActiveSupport::Concern
 
+      def self.included(mod)
+        # re-parse options at end of class definition
+        TracePoint.trace(:end) do |tp|
+          if mod == tp.self && mod.track_history_options
+            options_parser = Mongoid::History::Options.new(mod)
+            options = options_parser.parse(mod.track_history_options)
+            Mongoid::History.trackable_class_options[options_parser.scope] = options
+          end
+        end
+      end
+
       module ClassMethods
+        def track_history_options
+          @track_history_options
+        end
+
         def track_history(options = {})
+          # save options so they can be re-parsed at end of class definition
+          @track_history_options = options
+
           extend EmbeddedMethods
 
           options_parser = Mongoid::History::Options.new(self)

spec/integration/track_history_order_spec.rb

@@ -0,0 +1,18 @@
+require 'spec_helper'
+
+describe Mongoid::History::Tracker do
+  before :all do
+    class SpecModel
+      include Mongoid::Document
+      include Mongoid::History::Trackable
+
+      track_history on: :fields
+
+      field :foo
+    end
+  end
+
+  it 'should track all fields when field added after track_history' do
+    expect(SpecModel.tracked?(:foo)).to be true
+  end
+end