def in(set)
      @collection = @collection.collect do |item|
        item if set.include?(item.send(@field_name.to_sym))
      end.compact
    end
def in(set)
      @collection = @collection.collect do |item|
        item if set.include?(item.send(@field_name.to_sym))
      end.compact
    end
performs a set-inclusion test.

 Task.find(:id).in([3, 5, 9])