Fixing upserts involving SCI collections in MongoMapper

fix_single_collection_inheritance.rb

module MongoMapper
  module Plugins
    module Sci
      module ClassMethods
        # I override this method so that it works with upserts.
        def criteria_hash(criteria={})
          criteria = criteria.merge(:_type => name) if single_collection_inherited?
          super criteria
        end   
      end
    end
  end
end

I discuss this fix on my blog