Question

So I have a method in my video model:

def rank_sum(score, video)
 ...
 ...
end

that returns a value that I want to store in my database for the specific video in the rank_sum column.

I also order my videos like so:

default_scope order('videos.rank_sum')

Now my question is how should I update the rank_sum column? With a callback? How often should I be updating this?

Answer 1

I’d recommend that you don’t create methods that clash with any of the automatically created attributes of your model, as this will interfere with built-in Rails functionality such as form generation.

You could simply rewrite your method as follows:

def update_rank_sum(score)
  new_rank = self.rank_sum + score # Or substitute whatever logic is necessary here.
  update_attribute :rank_sum, new_rank
end

This will update the value in the database (if necessary) each time the update_rank_sum method is called.

If you want to postpone the update of the rank_sum value until some later point, you can change the implementation as follows:

def update_rank_sum(score)
  new_rank = self.rank_sum + score # Or substitute whatever logic is necessary here.
  self.rank_sum = new_rank
end

But remember that you will need to call save on the Video instance to persist the change to the database.

Updating one field of a single row is likely to take your database in the order of milliseconds, so I wouldn’t worry too much about performance unless you know you will need to handle 10s or 100s of changes per second.