Raise more describprive error when can't reflect on association

lib/cocoon/view_helpers.rb

@@ -92,6 +92,7 @@ module Cocoon
     def create_object(f, association)
       assoc = f.object.class.reflect_on_association(association)
 
+      raise "Association #{association} doesn't exist on #{f.object.class}" unless assoc
       if assoc.class.name == "Mongoid::Relations::Metadata"
         conditions = assoc.respond_to?(:conditions) ? assoc.conditions.flatten : []
         assoc.klass.new(*conditions)

spec/cocoon_spec.rb

@@ -191,6 +191,10 @@ describe Cocoon do
         result = @tester.create_object(stub(:object => Comment.new), :post)
         result.should be_a Post
       end
+
+      it "should raise error if cannot reflect on association" do
+        expect { @tester.create_object(stub(:object => Comment.new), :not_existing) }.to raise_error /exist/
+      end
     end
 
     context "get_partial_path" do