Use association build methods instead of assoc.klass.new. This avoids mass-assignment errors and other misbehaviors around attribute accessibility.

lib/cocoon/view_helpers.rb

@@ -84,9 +84,13 @@ module Cocoon
     # will create new Comment with author "Admin"
 
     def create_object(f, association)
-      assoc      = f.object.class.reflect_on_association(association)
-      conditions = assoc.respond_to?(:conditions) ? assoc.conditions.flatten : []
-      new_object = assoc.klass.new(*conditions)
+      assoc = f.object.class.reflect_on_association(association)
+
+      if assoc.collection?
+        f.object.send(association).build
+      else
+        f.object.send("build_#{association}")
+      end
     end
 
     def get_partial_path(partial, association)

spec/cocoon_spec.rb

@@ -175,11 +175,16 @@ describe Cocoon do
       end
     end
 
-    context "association with conditions" do
-      it "should create correct association" do
+    context "create_object" do
+      it "should create correct association with conditions" do
         result = @tester.create_object(@form_obj, :admin_comments)
         result.author.should == "Admin"
       end
+
+      it "should create correct association for belongs_to associations" do
+        result = @tester.create_object(stub(:object => Comment.new), :post)
+        result.should be_a Post
+      end
     end
 
     context "get_partial_path" do

spec/dummy/app/models/comment.rb

@@ -1,3 +1,5 @@
 class Comment < ActiveRecord::Base
   belongs_to :post
+
+  attr_protected :author
 end