Calling RESTful 'scoped' INDEX when params[:parent.id] is not present?

Jordon,
    I have a VERY hierchical database and am using a RESTful
approach. Normally, when users' enter the system they'll cascade from
level to level in the database and thus the hierchical information
such as child.parent.grandchild_id are always available to the RESTful
routes.
However, when I attempt to simply call a middle level table.index I
obviously fail, because the parent_id is not available to pass to the
RESTful route.
I'd like to have a 'safety mechanism' built into my code that handles
this, and provide the pieces that describe the problem.
    Here is an example of my before_filter event and the way I use it
to call my index:

def find_parent
@find_parent || @project = Project.find(params[:project_id])
end

Here is a typical table.index action:

def index
@portal_pages = Paginator.new self, @project.portals.count, 3,
params[:page]
@portals = @project.portals.find :all,
:order => 'description',
:limit => @portal_pages.items_per_page,
:offset => @portal_pages.current.offset
respond_to do |format|
format.html # index.rhtml
format.xml { render :xml => @portals.to_xml }
end
end

Would it be reasonable to make the INDEX code something like this;

def index
if @project.nil?
@portal_pages = Paginator.new self, Portal.count, 3, params[:page]
@portals = Portal.find :all,
:order => 'description',
:limit => @portal_pages.items_per_page,
:offset => @portal_pages.current.offset
else
@portal_pages = Paginator.new self, @project.portals.count, 3,
params[:page]
@portals = @project.portals.find :all,
:order => 'description',
:limit => @portal_pages.items_per_page,
:offset => @portal_pages.current.offset
end
respond_to do |format|
format.html # index.rhtml
format.xml { render :xml => @portals.to_xml }
end
end

    Fundumentally, what 'blows my mind' is that the 'find_parent'
'before_filter' event ALWAYS has to fire and if no parent exists, I
can't see how to return a '@project.nil' or whatever is needed to feed
the if, else routine (above)?

Thank you,
Kathy