Hi
If you have users and posts
You can User.find(blah blah) and Post.find(blah blah) - in both cases the relevant tables will be searched.
If you want to return users that have made posts in the last 10 minutes (assuming posts have user_id)
User.find(:all, :conditions => [‘posts.created_at > ?’,10.minutes.ago], :include => 'posts])
You will only get the users who posted in the last 10 minutes.
If you have @abc and you want to search the table associated with the records in @abc you can (assuming @abc is a collection of activerecord objects) do
@abc.first.class.name.classify.constantize.find(blah blah)
What this will do is get the class, from one of the elements in the collection. If @abc was a collection of users then @abc.first.class.name.classify.constantize
will be User. Before the constantize it will be “User”.
Anyway, thats about all I can say and probably all I should say - not really an expert here 
regards
ivor
