Extract complicated runner code into private class

Factory#run was getting confusing - it added callbacks to the proxy,
iterated over all attributes and operated on the proxy depending on if
there were overridtes, mutated the overrides hash, and then operated on
the mutated hash afterwards. This logic has been extracted into a
private Factory::Runner class. The goal is to make it easier to
understand everything that's occurring in order to instantiate and build
the class.

lib/factory_girl/factory.rb

@@ -46,28 +46,16 @@ def define_trait(trait)
 
     def run(proxy_class, overrides, &block) #:nodoc:
       ensure_compiled
-      proxy = proxy_class.new(build_class)
-      callbacks.each { |callback| proxy.add_callback(callback) }
-      overrides = overrides.symbolize_keys
-
-      attributes.each do |attribute|
-        factory_overrides = overrides.select { |attr, val| attribute.aliases_for?(attr) }
-        if factory_overrides.empty?
-          attribute.add_to(proxy)
-        else
-          factory_overrides.each do |attr, val|
-            if attribute.ignored
-              proxy.set_ignored(attr, val)
-            else
-              proxy.set(attr, val)
-            end
-
-            overrides.delete(attr)
-          end
-        end
-      end
-      overrides.each { |attr, val| proxy.set(attr, val) }
-      result = proxy.result(@to_create_block)
+
+      runner_options = {
+        :attributes  => attributes,
+        :callbacks   => callbacks,
+        :to_create   => @to_create_block,
+        :build_class => build_class,
+        :proxy_class => proxy_class
+      }
+
+      result = Runner.new(runner_options).run(overrides)
 
       block ? block.call(result) : result
     end
@@ -200,5 +188,73 @@ def parent
       return unless @parent
       FactoryGirl.factory_by_name(@parent)
     end
+
+    class Runner
+      def initialize(options = {})
+        @attributes  = options[:attributes]
+        @callbacks   = options[:callbacks]
+        @to_create   = options[:to_create]
+        @build_class = options[:build_class]
+        @proxy_class = options[:proxy_class]
+
+        @overrides   = {}
+      end
+
+      def run(overrides = {})
+        @overrides = overrides.symbolize_keys
+
+        apply_callbacks
+        apply_attributes
+        apply_remaining_attributes
+
+        proxy.result(@to_create)
+      end
+
+      private
+
+      def apply_callbacks
+        @callbacks.each do |callback|
+          proxy.add_callback(callback)
+        end
+      end
+
+      def apply_attributes
+        @attributes.each do |attribute|
+          if overrides_for_attribute(attribute).any?
+            handle_overrides(attribute)
+          else
+            handle_attribute_without_overrides(attribute)
+          end
+        end
+      end
+
+      def overrides_for_attribute(attribute)
+        @overrides.select { |attr, val| attribute.aliases_for?(attr) }
+      end
+
+      def handle_overrides(attribute)
+        overrides_for_attribute(attribute).each do |attr, val|
+          if attribute.ignored
+            proxy.set_ignored(attr, val)
+          else
+            proxy.set(attr, val)
+          end
+
+          @overrides.delete(attr)
+        end
+      end
+
+      def handle_attribute_without_overrides(attribute)
+        attribute.add_to(proxy)
+      end
+
+      def apply_remaining_attributes
+        @overrides.each { |attr, val| proxy.set(attr, val) }
+      end
+
+      def proxy
+        @proxy ||= @proxy_class.new(@build_class)
+      end
+    end
   end
 end