Allow created_at to be set when using build_stubbed

Closes thoughtbot#390

lib/factory_girl/strategy/stub.rb

@@ -22,15 +22,12 @@ def next_id
 
       def stub_database_interaction_on_result(result_instance)
         result_instance.id = next_id
+
         result_instance.instance_eval do
           def persisted?
             !new_record?
           end
 
-          def created_at
-            @created_at ||= Time.now
-          end
-
           def new_record?
             id.nil?
           end
@@ -55,6 +52,17 @@ def update_attribute(*args)
             raise 'stubbed models are not allowed to access the database'
           end
         end
+
+        created_at_missing_default = result_instance.respond_to?(:created_at) && !result_instance.created_at
+        result_instance_missing_created_at = !result_instance.respond_to?(:created_at)
+
+        if created_at_missing_default || result_instance_missing_created_at
+          result_instance.instance_eval do
+            def created_at
+              @created_at ||= Time.now
+            end
+          end
+        end
       end
     end
   end

spec/acceptance/build_stubbed_spec.rb

@@ -102,3 +102,38 @@
     end.should == expected
   end
 end
+
+describe "defaulting `created_at`" do
+  include FactoryGirl::Syntax::Methods
+
+  before do
+    define_model('ThingWithTimestamp', created_at: :datetime)
+    define_model('ThingWithoutTimestamp')
+
+    FactoryGirl.define do
+      factory :thing_with_timestamp
+      factory :thing_without_timestamp
+    end
+
+    Timecop.freeze Time.now
+  end
+
+  after { Timecop.return }
+
+  it "defaults created_at for objects with created_at" do
+    build_stubbed(:thing_with_timestamp).created_at.should == Time.now
+  end
+
+  it "adds created_at to objects who don't have the method" do
+    build_stubbed(:thing_without_timestamp).should respond_to(:created_at)
+  end
+
+  it "allows overriding created_at for objects with created_at" do
+    build_stubbed(:thing_with_timestamp, created_at: 3.days.ago).created_at.should == 3.days.ago
+  end
+
+  it "doesn't allow setting created_at on an object that doesn't define it" do
+    expect { build_stubbed(:thing_without_timestamp, :created_at => Time.now) }.to
+      raise_error(NoMethodError, /created_at=/)
+  end
+end