Ensure that callbacks are triggered once for implicit traits

Closes thoughtbot#393

lib/factory_girl/declaration_list.rb

@@ -21,7 +21,7 @@ def overridable
     end
 
     def attributes
-      AttributeList.new(@name).tap do |list|
+      @attributes ||= AttributeList.new(@name).tap do |list|
         to_attributes.each do |attribute|
           list.define_attribute(attribute)
         end

spec/acceptance/traits_spec.rb

@@ -645,3 +645,27 @@ def initialize(name)
     it_should_behave_like "assigning data from traits"
   end
 end
+
+describe "implicit traits containing callbacks" do
+  before do
+    define_model("User", value: :integer)
+
+    FactoryGirl.define do
+      factory :user do
+        value 0
+
+        trait :trait_with_callback do
+          after(:build) {|user| user.value += 1 }
+        end
+
+        factory :user_with_trait_with_callback do
+          trait_with_callback
+        end
+      end
+    end
+  end
+
+  it "only runs the callback once" do
+    FactoryGirl.build(:user_with_trait_with_callback).value.should == 1
+  end
+end

spec/factory_girl/declaration_list_spec.rb

@@ -25,10 +25,6 @@
     attribute_list.should have_received(:define_attribute).with(static_attribute_2)
     attribute_list.should have_received(:define_attribute).with(dynamic_attribute_1)
   end
-
-  it "creates a new attribute list upon every invocation" do
-    subject.attributes.should_not == subject.attributes
-  end
 end
 
 describe FactoryGirl::DeclarationList, "#declare_attribute" do