adding data from the model

Hi, I have a form that I need to add default data when is passed into the DB..
for example I have a create post, were the user can add tittle, text etc..
but I need to add who created so is on record etc
I will like to know how will I do this from the model, since I think that is the right place to do this..
on php I will just add hiden fields to the form and pass the variables but there should be a nice way in rails..

thanks

Post.create( params[:post].merge( :user_id => session[:user_id] ) )