# Can i get the id of a new object before saving it/as i save? **URL:** \ **Category:** Rails\ **Created:** [January 24, 2008, 1:30pm UTC](https://www.ruby-forum.com/t/can-i-get-the-id-of-a-new-object-before-saving-it-as-i-save/127037 "2008-01-24T13:30:22Z")\ **Posts on this page:** 4\ **Page:** 1
**Author:** ![cmaxvv](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/c/85f322/32.png) [@cmaxvv](https://www.ruby-forum.com/u/cmaxvv)\ **Post date:** [January 24, 2008, 1:30pm UTC](https://www.ruby-forum.com/t/can-i-get-the-id-of-a-new-object-before-saving-it-as-i-save/127037/1 "2008-01-24T13:30:22Z")
I have a batch-add page for users that needs to auto-generate a unique login name for each user. What i want to do is set their login name to be “#{first\_name}\_#{id}”. The problem is - i can’t see the id until i save the object, but i don’t want to save it until i’ve set the login. Is there any way around this? Can i start creating an object and tell it to use the id that will created when it’s saved? Or do i need to save it, get the id and then save it again? thanks max ---
**Author:** ![cmaxvv](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/c/85f322/32.png) [@cmaxvv](https://www.ruby-forum.com/u/cmaxvv)\ **Post date:** [January 24, 2008, 1:57pm UTC](https://www.ruby-forum.com/t/can-i-get-the-id-of-a-new-object-before-saving-it-as-i-save/127037/2 "2008-01-24T13:57:41Z")
On 24/01/2008, Max W. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > I have a batch-add page for users that needs to auto-generate a unique > login name for each user. What i want to do is set their login name to > be “#{first\_name}\_#{id}”. The problem is - i can’t see the id until i > save the object, but i don’t want to save it until i’ve set the login. There is no id until you save it to the database. Rails (and any other application) cannot tell what the next available id in the database will be. You could try to determine the highest used id in the database table and increment this id but this does not guarantee that this id id is not used by another user in the meantime and is no more available. -Thomas – Thomas P. removed\_email\_address@domain.invalid removed\_email\_address@domain.invalid Büro: 030 - 830 353 88 mobil: 0176 - 75 03 03 04 Privat: 030 - 49 78 37 06 > **[thopre](https://thopre.wordpress.com/)** > > Entwicklungen für die Bühne des Lebens [http://www.thopre.com/](http://www.thopre.com/) ---
**Author:** ![cmaxvv](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/c/85f322/32.png) [@cmaxvv](https://www.ruby-forum.com/u/cmaxvv)\ **Post date:** [January 24, 2008, 2:05pm UTC](https://www.ruby-forum.com/t/can-i-get-the-id-of-a-new-object-before-saving-it-as-i-save/127037/3 "2008-01-24T14:05:20Z")
Hi, There is one filter called “after\_save”, it will invoke a method (let’s say a foo method) after model.save! succeed (here, you already know the id). I figure you can use the foo method to update the login name"#{first\_name}\_#{id}" column in database. However, I still take this is somehow wired… it updates database twice… unless you really have the reason to do this. P.s. I dont think there is a good way to know the id beforehand unless you really stored this user, cuz even you are able to know the last id in database, you still cant guarantee that there is nobody registering just one second before you! Br, MyST On Jan 24, 2:30 pm, Max W. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) ---
**Author:** ![cmaxvv](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/c/85f322/32.png) [@cmaxvv](https://www.ruby-forum.com/u/cmaxvv)\ **Post date:** [January 24, 2008, 2:30pm UTC](https://www.ruby-forum.com/t/can-i-get-the-id-of-a-new-object-before-saving-it-as-i-save/127037/4 "2008-01-24T14:30:37Z")
myst\_tt wrote: > Hi, > There is one filter called “after\_save”, it will invoke a method > (let’s say a foo method) after model.save! succeed (here, you already > know the id). I figure you can use the foo method to update the login > name"#{first\_name}\_#{id}" column in database. Thanks myst\_tt, i don’t want to use a callback because the situation’s a little more complicated than i described: by default, user’s logins are set the same as their email address, but emails aren’t forced to be unique (since a group of users can share an email). So, i actually only need to autogenerate a login some of the time (a minority of cases actually). What i’ve ended up doing (and it seems dirty) is make a temporary probably-unique login by combining first name and time since epoch in microseconds - rails/mysql is unlikely to ever manage 2 transactions in one microsecond so this should be fine. Then, after saving, i call update\_attributes to set it to firstname+id, which is guaranteed to be unique and also more memorable than firstname+microseconds\_since\_epoch. Like i say, i’m not very happy with this though.