I have a collection of Post objects and I want to be able to sort them based on these conditions:

• First, by category (news, events, labs, portfolio, etc.)
• Then by date, if date, or by position, if a specific index was set for it

Some posts will have dates (news and events), others will have explicit positions (labs, and portfolio).

I want to be able to call posts.sort!, so I've overridden <=>, but am looking for the most effective way of sorting by these conditions. Below is a pseudo method:

def <=>(other)
  # first, everything is sorted into 
  # smaller chunks by category
  self.category <=> other.category

  # then, per category, by date or position
  if self.date and other.date
    self.date <=> other.date
  else
    self.position <=> other.position
  end
end

It seems like I'd have to actually sort two separate times, rather than cramming everything into that one method. Something like sort_by_category, then sort!. What is the most ruby way to do this?