Question

How do you work with the old values of a record being updated?

For instance in the following code block how would I run a query using the previous winner_id field after I determine that it has indeed changed?

if self.winner_id_changed?
   old_value = self.changed_attributes
   User.find(old_value)
   #do stuff with the old winner....
end

An example output of self.changed_attributes would be:

{“winner_id”=>6}

Do I really have to convert this to a string and parse out the value in order to perform a query on it? old_value[:winner_id] doesn’t seem to do anything.

Answer 1

Use where instead of find, and the following inject method on changes to generate the desired hash:

if self.winner_id_changed?
   old_value = self.changes.inject({}) { |h, (k,v)| h[k] = v.first }
   old_user = User.where(old_value)
   #do stuff with the old user....
end

You can also use ActiveRecord dirty methods such as:

self.winner_id_was

to get specific attribute’s old value. Full documentation may be found here.