Optimization: Optimize CollectionDecorator Enumerable behavior (#707)

* Optimization: Prevent duplicate iteration

* Don't delegate Enumerable instance methods since CollectionDecorator includes Enumerable itself

lib/draper/collection_decorator.rb

@@ -15,7 +15,7 @@ module Draper
     #   to each item's decorator.
     attr_accessor :context
 
-    array_methods = Array.instance_methods - Object.instance_methods
+    array_methods = Array.instance_methods - Object.instance_methods - Enumerable.instance_methods
     delegate :==, :as_json, *array_methods, to: :decorated_collection
 
     # @param [Enumerable] object
@@ -42,6 +42,23 @@ module Draper
       @decorated_collection ||= object.map{|item| decorate_item(item)}
     end
 
+    # Optimization to prevent unnecessary iteration (useful for larger collections).
+    # Iterates over the collection, decorating objects as it goes. If the decorated collection is
+    # already set, iterate over it instead.
+    def each(&block)
+      if @decorated_collection
+        @decorated_collection.each(&block)
+      else
+        @decorated_collection = []
+        object.each do |item|
+          decorated_item = decorate_item(item)
+          @decorated_collection << decorated_item
+
+          block.call(decorated_item)
+        end
+      end
+    end
+
     delegate :find, to: :decorated_collection
 
     def to_s

spec/draper/collection_decorator_spec.rb

@@ -288,5 +288,38 @@ module Draper
       end
     end
 
+    describe "#each" do
+      it "iterates over the collection, decorating as it goes" do
+        collection = [Product.new]
+        decorator = CollectionDecorator.new(collection)
+
+        expect(collection).to_not receive(:map)
+        decorator.each { |product| product.decorated? }
+        expect(decorator.instance_variable_get(:@decorated_collection)[0]).to be_decorated
+      end
+
+      it "uses decorated_collection if already set" do
+        decorated_collection = double(:decorated_collection)
+        decorator = CollectionDecorator.new([])
+        decorator.instance_variable_set(:@decorated_collection, decorated_collection)
+
+        expect(decorated_collection).to receive(:each)
+
+        decorator.each
+      end
+    end
+
+    describe "Enumerable methods" do
+      it "doesn't delegate Enumerable methods to its decorated collection" do
+        decorated_collection = double(:decorated_collection)
+        decorator = CollectionDecorator.new([])
+        decorator.instance_variable_set(:@decorated_collection, decorated_collection)
+
+        expect(decorated_collection).to_not receive(:map)
+
+        decorator.map
+      end
+    end
+
   end
 end