example.rb

def update
  # fetch @conversation having its rank updated.
  #
  conversations = @discussion.conversations.order(:rank).to_a
  
  # Move conversation to new position by inserting in array at rank and deleting it from previous position.
  conversations.insert(@conversation.rank, conversations.delete_at(@conversation))
  
  # Update all conversation ranks to correspond to new array order.
  # Very inefficient as this uses a n+1 query. You could optimize to use update_all
  # or something of the sort.
  conversations.each_with_index do |c, i|
    c.update_attributes(rank: i)
  end
end