Allow factories with traits to be used in associations

This allows defining associations with factories using traits by slicing
up the hash passed to association. Passing an array to `:factory` will
use the first item in the array as the factory, with any subsequent
symbols as traits.

Here's an example:

    factory :post do
      association :author, factory: [:user, :admin], name: 'John Doe'
    end

Closes thoughtbot#395

lib/factory_girl/attribute/association.rb

@@ -13,7 +13,10 @@ def initialize(name, factory, overrides)
       def to_proc
         factory   = @factory
         overrides = @overrides
-        -> { association(factory, overrides) }
+        traits_and_overrides = [factory, overrides].flatten
+        factory_name = traits_and_overrides.shift
+
+        -> { association(factory_name, *traits_and_overrides) }
       end
 
       def association?

lib/factory_girl/evaluator.rb

@@ -23,10 +23,13 @@ def initialize(build_class, build_strategy, overrides = {})
 
     delegate :new, to: :@build_class
 
-    def association(factory_name, overrides = {})
+    def association(factory_name, *traits_and_overrides)
+      overrides = traits_and_overrides.extract_options!
       strategy_override = overrides.fetch(:strategy) { :create }
 
-      runner = FactoryRunner.new(factory_name, strategy_override, [overrides.except(:strategy)])
+      traits_and_overrides += [overrides.except(:strategy)]
+
+      runner = FactoryRunner.new(factory_name, strategy_override, traits_and_overrides)
       @build_strategy.association(runner)
     end
 

spec/acceptance/traits_spec.rb

@@ -669,3 +669,32 @@ def initialize(name)
     FactoryGirl.build(:user_with_trait_with_callback).value.should == 1
   end
 end
+
+describe "traits used in associations" do
+  before do
+    define_model("User", admin: :boolean, name: :string)
+    define_model("Post", author_id: :integer) do
+      belongs_to :author, class_name: 'User'
+    end
+
+    FactoryGirl.define do
+      factory :user do
+        admin false
+
+        trait :admin do
+          admin true
+        end
+      end
+
+      factory :post do
+        association :author, factory: [:user, :admin], name: 'John Doe'
+      end
+    end
+  end
+
+  it "allows assigning traits for the factory of an association" do
+    author = FactoryGirl.create(:post).author
+    author.should be_admin
+    author.name.should == 'John Doe'
+  end
+end