The code that I was emulating is:

# products_controller.rb
def update
  params[:product][:category_ids] ||= []
  #...
end

The problem I was having is that

params[:product]

was nil as the http post sent nothing because I only had checkboxes on my form. Defaulting to

[]

wasn’t valid for the

update_attributes

method.

The hack that I finally came up with was like this:

# products_controller.rb
def update
  if params[:product].nil?
    @products.categories = []
  else
    @products.update_attributes(params[:product])
  end
  redirect_to :action => :index
end

So it turns out, in this HABTM scenario, one can address the entire list of categories with an array.