will_paginate and :order


I would like to code a kind of 'forum'.

I'm using will_paginate and I would like my topics to be order by the
newest answer by topic.

For the moment I have:
- in my controller:
@categorie = Categoriemessage.find(params[:id])
@souscategories = Categoriemessage.find(:all, :conditions => ["parent_id
= ?", params[:id]])
@sujets = Message.paginate :per_page => 10, :page => params[:page],
                            :conditions => ["sujet_id = ? AND
categorie_id = ?", '0',params[:id]]

- in my helper:
def datedernierereponse(sujet_id)
  m = Message.find(:first, :conditions => ["sujet_id = ?", sujet_id],
:order => "datecreation desc")
  if m.nil?
    m = Message.find(sujet_id)

Like this, my application works.
@categorie is the current category of the list
@souscategories are the children's categories of @categorie.
@sujets is the list of topics showed in the view.
datededernierereponse(sujet_id) is a function giving the date of the
newest answer of a topic.

I don't how to do:
- order @sujets by the newest answer.
- I would like @sujets to include the topics of the @souscategories,
where 'categorie_id' = @souscategories.id

My english isn't so good, don't hesitate to ask for more explanation or
a reformulation.

Thanks !

The second problem is solved (include the topics of @souscategories in
my request)

Here is the code:

@souscategories = Categoriemessage.find(:all, :conditions => ["parent_id
= ?", params[:id]])
categories = [@categorie] + @souscategories
@sujets = Message.paginate :per_page => 10, :page => params[:page],
:conditions => {:sujet_id => '0', :categorie_id => categories}

I still don't know how to order the request by the date of the newest
(As i mentionned, I have a helper method giving the date of the newest
answer, it just need the id of the topic.

I can give more information if it's needed.