Properly proxy to Enumerable or AR::Finder method.

Decorated associations implement Enumerable, but they also should
emulate the AR::Finder stuff.

    @person = PersonDecorator.find(params[:person_id])
    @pet = @person.pets.find(params[:id])

The problem is, pets delegated to Enumerable always, instead of
to the finder method. This patch makes it delegate to Enumerable if
a block is given, and Finder if not.

Fixes #214.

lib/draper/base.rb

@@ -64,6 +64,7 @@ module Draper
         orig_association = model.send(association_symbol)
 
         return orig_association if orig_association.nil?
+        return decorated_associations[association_symbol] if decorated_associations[association_symbol]
 
         orig_association = orig_association.send(options[:scope]) if options[:scope]
 
@@ -79,7 +80,7 @@ module Draper
                   orig_association.class
                 end
 
-        "#{klass}Decorator".constantize.decorate(orig_association, options)
+        decorated_associations[association_symbol] = "#{klass}Decorator".constantize.decorate(orig_association, options)
       end
     end
 
@@ -303,5 +304,9 @@ module Draper
         model.class.reflect_on_association(association)
       end
     end
+
+    def decorated_associations
+      @decorated_associations ||= {}
+    end
   end
 end

lib/draper/decorated_enumerable_proxy.rb

@@ -1,3 +1,4 @@
+require 'active_support/core_ext/object/blank'
 module Draper
   class DecoratedEnumerableProxy
     include Enumerable
@@ -13,6 +14,16 @@ module Draper
     end
     alias_method :to_ary, :decorated_collection
 
+    def find(ifnone_or_id = nil, &blk)
+      if block_given?
+        source.find(ifnone_or_id, &blk)
+      else
+        obj = decorated_collection.first
+        return nil if obj.blank?
+        obj.class.find(ifnone_or_id)
+      end
+    end
+
     def method_missing (method, *args, &block)
       @wrapped_collection.send(method, *args, &block)
     end

spec/draper/base_spec.rb

@@ -178,6 +178,23 @@ describe Draper::Base do
         subject.previous_version.should be_nil
       end
     end
+
+    context "#find" do
+      before(:each){ subject.class_eval{ decorates_association :similar_products } }
+      context "with a block" do
+        it "delegates to #each" do
+          subject.similar_products.source.should_receive :find
+          subject.similar_products.find {|p| p.title == "title" }
+        end
+      end
+
+      context "without a block" do
+        it "calls a finder method" do
+          subject.similar_products.source.should_not_receive :find
+          subject.similar_products.find 1
+        end
+      end
+    end
   end
 
   context('.decorates_associations') do