Question

I want to create a hash that combines the creating user’s user_id + the record’s ID to make a MD5 hash, but only on record creation. (Reasons are long-winded but this extracts it).

I am trying:

class BlogPost < ActiveRecord::Base  

  after_create :hash_this

  private

  def hash_this
    self.info_md5_hashed = (id.to_str + creator_user_id).my_MD5_hash_method
  end

end

How can I make sure that the info_md5_hashed field actually gets saved to the database?

If I use before_create I would assume that the ID is not yet available? 🙁
If I use after_create I would assume that the ID is now available
– but do I need to do another save somehow to store the newly calculated info_md5_hashed field value?

Answer 1

Yes, you will have to save the record twice, since ID generation happens in the database. The only way around that is to pre-sequence an ID, but there’s almost certainly no point and you should just accept that you will have to save the record twice 😉

Just call the second save from inside your after_create hook- it’s ok to do this (i.e. will not be a looping recurrence issue) because the second save won’t re-trigger that hook again 😉

n.b. You could always base the hash on something you know to be unique, such as the username or email, if you want to avoid a double-save.