:o
Do u did before_filtter for the workut to load the activity???
if not so do that in this way
i assume the fallowing relation ship b/w models depending on ur routes
Model Workout
has_many :activities
Model Activity
belongs_to :workout
In the workout controller i assume
def new
@workout= Workout.new
respond_to do |format|
format.html # new.html.erb
format.xml { render :xml => @workout}
end
end
def create
@workout= Workout.new(params[:workout])
respond_to do |format|
if @workout.save
format.html { redirect_to(@workout) }
format.xml { render :xml => @workout, :status => :created,
:location => @workout}
else
format.html { render :action => “new” }
format.xml { render :xml => @workout.errors, :status =>
:unprocessable_entity }
end
end
end
in ur activity controller
before_filter :find_workout
def new
@activity= Activity.new
end
def create
@activity = Activity.new(params[:activity])
if (@workout.activities << @activity)
redirect_to workout_url(@workout)
else
render :action => :new
end
end
private
def find_workout
@workout_id = params[:workout_id]
return(redirect_to(workouts_url)) unless @workout_id
@workout= Workout.find(@workout_id)
end
and ur view file for activity/new
<% form_for [@workout, @activity] do |form| %>
Add an Activity
<%= render :partial => ‘form’, :object => form %>
<%= submit_tag "Create" %>
<% end %>
*and the link for the new activity will be
<%= link_to “Add Activity”, new_workout_activity_url(@workout) %>
*
*and ur routes
map.resources :workouts do |workout|
workout.resources :activities
end*
If still any issues :o let us know 

Bala Kishore