+1  A: 

I think that the problem is you've not got @list defined anywhere when you're using the render :partial with a :collection.

The system is looking for @list to match the list[] when you call f.check_box

you could set @list = list in your partial to get around that. I suppose.

Tim Harding
A: 

Tim's answer is correct, but I'd probably avoid extracting the partial within the form_for loop altogether. I suppose it's a matter of style, but I think the confusion here isn't really worth the cleanup that the partial represents in this case. I'd probably write a partial that included the whole form.

Cameron Price