Multi-model forms


I'm trying to implement multi-model forms.  Specifically a form containing the following relationships ...

	parent -> children

	child  -> grandchildren

	[ NOTE : "->" denotes a one to many relationship / association - the '>' points to the many ]

Recipe #13 in Advanced Rails Recipes does a great job of explaining how to implement the parent -> children relationship using Project and Tasks.  Specifically ...

		project -> tasks  (using my "->" nomenclature)

I'd like to include an additional model - for example adding "Participants" to the above example ...

		project -> tasks

		task    -> participants

Where I'm getting stuck is what to use for the 'prefix' (using the terminology of Recipe #13) - or what name to use to refer to 'participants', in such a way that the params hash is properly populated for 'auto-population' by the controller?

Using the Project/Task metaphor and extending it to include the relationship that a Task has many Participants, the 'task' partial would look something like this (not in its final form - but early on in the development of a +2 model multi-model form.  Also note the '??????') ...
<% fields_for "project[task_attributes][]", task do |f| %>


		Task: <%= f.text_field :name %>


	<% 3.times { } %>

	<% for participant in task.participants %>

		<% fields_for "project[???????]", participant do |p_form| %>


				Participant Name: <%= p_form.text_field :name	%>


		<% end %>

	<% end %>

<% end %>
I'm assuming that a Task can have at most 3 participants, like what was done in the early iterations of Ryan Bate's original screen-cast.

I'm trying to figure out how to name the 'grandchildren' such that the params hash is correctly populated ?   Specifically - what to put in place of the '?????' in this line ...

	<% fields_for "project[???????]", participant do |p_form| %>

If the solution doesn't involve replacing the '??????' above - please point me in the right direction by providing as much detail / info as you have time for.

I'm grateful for any help / guidance you can provide.