Overrides become methods defined on each instance of the evaluator

There were some big issues with trying to undefine specific methods on
the Evaluator. After investigating maybe inheriting from BasicObject (or
ActiveSupport::BasicObject since BasicObject is 1.9+), that turned out
to be too much of a pain because it undefines almost everything,
including class and a handful of other methods necessary for Evaluator
to work properly.

The second solution was to undefine all private methods. The problem is,
when other libraries defining methods (private or otherwise) on Object
are loaded *after* factory girl, those methods get added and Evaluator
sees those methods on Object. So, that solution didn't directly work either.

This commit removes undefining methods (the sole reason of which was to
capture with method_missing and process ourselves, returning the
override or cached value) and instead introduces a new concept -
iterating over each override and defining it as a method on the
evaluator INSTANCE. This means that overrides don't collide because
they're on the instance and we don't have to worry about undefining
methods so that method_missing kicks in. This is the most stable and
guaranteed way to get this to work because the overrides are applied to
each instance at runtime.

Closes #279, #285

lib/factory_girl/evaluator.rb

@@ -11,13 +11,21 @@ module FactoryGirl
         end
       end
     end
-    undef_method(:id) if method_defined?(:id)
+
+    private_instance_methods.each do |method|
+      undef_method(method) unless method =~ /^__|initialize/
+    end
 
     def initialize(build_strategy, overrides = {})
       @build_strategy    = build_strategy
       @overrides         = overrides
       @cached_attributes = overrides
 
+      singleton = class << self; self end
+      @overrides.each do |name, value|
+        singleton.send :define_method, name, lambda { value }
+      end
+
       @build_strategy.add_observer(CallbackRunner.new(self.class.callbacks, self))
     end
 

spec/acceptance/attribute_existing_on_object_spec.rb

@@ -19,3 +19,50 @@ describe "declaring attributes on a Factory that are private methods on Object"
   its(:link)   { should == "http://example.com" }
   its(:sleep)  { should == -5 }
 end
+
+describe "assigning overrides that are also private methods on object" do
+  before do
+    define_model("Website",  :format => :string, :y => :integer, :more_format => :string, :some_funky_method => :string)
+
+    Object.class_eval do
+      private
+      def some_funky_method(args)
+      end
+    end
+
+    FactoryGirl.define do
+      factory :website do
+        more_format { "format: #{format}" }
+      end
+    end
+  end
+
+  after do
+    Object.send(:undef_method, :some_funky_method)
+  end
+
+  subject { FactoryGirl.build(:website, :format => "Great", :y => 12345, :some_funky_method => "foobar!") }
+  its(:format)            { should == "Great" }
+  its(:y)                 { should == 12345 }
+  its(:more_format)       { should == "format: Great" }
+  its(:some_funky_method) { should == "foobar!" }
+end
+
+describe "accessing methods from the instance within a dynamic attribute that is also a private method on object" do
+  before do
+    define_model("Website", :more_format => :string) do
+      def format
+        "This is an awesome format"
+      end
+    end
+
+    FactoryGirl.define do
+      factory :website do
+        more_format { "format: #{format}" }
+      end
+    end
+  end
+
+  subject           { FactoryGirl.build(:website) }
+  its(:more_format) { should == "format: This is an awesome format" }
+end