getting errors and params back to view

I asked this question here:
but let me try to put it more succinctly.

Below I show a controller action controller meant to save a new item
with its association. The page with the orignating form has displays of
other instances of the same model type and parent and associations

If a user enters information for a fposition with a field entry that
fails validation, I can't get the error messages and information back to
the view.

If I just render the view, the view seems to be missing all the instance
variables it needs. (instance variables only last one action I guess).
If I redirect to the view action that initially rendered the view, that
action creates a new object without the errors and partially completed
data from the first attempt.

def create_fposition
    @fposition =[:fposition])
    @formation = session[:formation]
    @coach = session[:coach]
#need help here
     if @fposition #! Help here please...need a condition !!!

@fposition will not be nil because it an Fposition object.(a new
Fposition object can be nil?) maybe you want to do @fposition.valid?


Can you use a before_filter to setup the common instance variables?
This way you can use render to display validation errors without
issues of the select boxes.

For example:

before_filter :setup :only =>
[:formations_fpositions, :create_fposition]

def setup
@formation = session[:formation]
@formation_fpositions = @formation.fpositions