Should fields_for for has_many association execute block even if the collection is empty?

Hi,

I had a problem where fields_for block was executed once, even if the
association collection was empty - child_form.object was nil of
course. Adding accepts_nested_attributes to parent model solved the
problem - block is now executed correctly for each instance of the
collection.

Is this proper behavior or a bug? I'm using Rails 2.3.4

Cheers,
Szymek