Question

I am seeing a very strange issue with a simple controller method. Either I am missing something fundamental or I am encountering a bug. My bet is on the former.

I have a Thing model with a ThingController.

A Thing has two variables, name and display, both strings.

ThingController (code below) has a method toggle_display, that toggles the contents of display between "on" and "off".

The problem is that when I call this action, Rails finds the correct Thing, but @thing.display is nil. When I check the database, the value in the ‘display’ column is correct.

The strange part is that when I uncomment the third line in the code below (i.e. access @thing.name before accessing @thing.display) then @thing.display is fine – it isn’t nil and it has the value I would expect. It’s as if @thing.display only gets initialized correctly after I access @thing.name.

Any idea why I would see this very strange behavior?

def toggle_display
  @thing = Thing.find(params[:id])

  # @thing.name

  if @thing.display
    @thing.toggle_display_in_model
    @thing.save
  end

  redirect_to things_url
end

Answer 1

The problem is that there’s already a method named “display” in Kernel which conflicts with ActiveRecord’s magic.

ActiveRecord defines the methods corresponding to the database fields in method_missing. So until method_missing is called, the methods don’t actually exist. When you call name on @thing, method_missing is called because there is no name method. However when you call display (without previously calling another method that does not exist), method_missing is not called because display is already defined in Kernel and this definition is executed. And since Kernel’s display method returns nil, you get nil.