Create one object and update another in one request

I see what you're asking so I'll try stepping you through it.

At the point the user clicks "make private" there is an object that has already been created, correct? Assuming so then simply pass that object id along with the "make private" command when the not logged in user is redirected to the login page. This means you should have a param list with something like the following: object_id => "1", method => "make_private"

Now you have a choice, you can either update the object within the check for valid login method you should already have by doing this:

if params[:object_id] && params[:method] == "make_private"
  #make object private
  redirect_to objects_path(params[:object_id])
else 
  #normal login redirect
end

or you can redirect in your login confirm method to the object's update method and from their redirect to the object's view page with something like the following:

def login_validation
  #normal login checking
  if params[:object_id] && params[:method] == "make_private"
    redirect_to object_path(params[:object_id])
  else
    #redirect to normal after login page
  end
end

then in your update you'll want:

def update
  #normal update stuff   
  redirect_to objects_path(params[:object_id])
end  

Obviously you'll need something that will detect the "make private" command in your update method but that shouldn't be too difficult. If you need help on that just comment here and I'll add the instructions for that as well.

Update: If you really wanted to be fancy you could pass the redirect path itself into the login function and then just redirect to whatever the login path was provided.

For example your make private button could look like this

<%= button_to "Make Private", object_path(:id=>object_id) %>

Then it would attempt to update, realize it had no logged in user, and redirect to login page like so:

def update
  if user_logged_in?
    #do normal update stuff
  else
    redirect_to login_path(:redirect_path => object_path(params[:id])
end

Then they'd fill out their normal information and when they clicked confirm it would have an additional parameter like so:

<%= button_to "Confirm", login_validation_path(<normal login params>,params[:redirect_path]) %>

Then inside your login_validation you'd do your normal validations and then redirect to whatever path you were given.

def login_validation
  #normal login stuffs
  if params[:redirect_path]
    redirect_to params[:redirect_path]
  else
    #normal redirect
end

That way your login could handle being called from multiple varying locations and correctly redirect to wherever you wanted it to.