# Single id column **URL:** \ **Category:** Rails\ **Created:** [July 22, 2006, 9:35am UTC](https://www.ruby-forum.com/t/single-id-column/99398 "2006-07-22T09:35:11Z")\ **Posts on this page:** 6\ **Page:** 1
**Author:** ![frebe73](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/f/5daacb/32.png) [@frebe73](https://www.ruby-forum.com/u/frebe73)\ **Post date:** [July 22, 2006, 9:35am UTC](https://www.ruby-forum.com/t/single-id-column/99398/1 "2006-07-22T09:35:11Z")
Is it true that Rails needs a single column to uniquely identify a row in table? If yes, doesn’t this limit the database design and disables the use of Rails for legacy databases? /Fredrik ---
**Author:** ![frebe73](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/f/5daacb/32.png) [@frebe73](https://www.ruby-forum.com/u/frebe73)\ **Post date:** [July 22, 2006, 12:41pm UTC](https://www.ruby-forum.com/t/single-id-column/99398/2 "2006-07-22T12:41:24Z")
“Fredrik B.” [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote in message news:removed\_email\_address@domain.invalid… > Is it true that Rails needs a single column to uniquely identify a row > in table? If yes, doesn’t this limit the database design and disables > the use of Rails for legacy databases? > > /Fredrik rails doesn’t currently support composite primary keys. Workarounds have been discussed on the list in the past but I don’t have any specific posts for you. Check the archives ---
**Author:** ![frebe73](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/f/5daacb/32.png) [@frebe73](https://www.ruby-forum.com/u/frebe73)\ **Post date:** [July 23, 2006, 3:55am UTC](https://www.ruby-forum.com/t/single-id-column/99398/3 "2006-07-23T03:55:51Z")
It is true except for the habm join tables those do not need them since they are never instanciated as AR objects.(at least thats what my memory is telling me.) On 7/22/06, Fredrik B. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > removed\_email\_address@domain.invalid > [http://lists.rubyonrails.org/mailman/listinfo/rails](http://lists.rubyonrails.org/mailman/listinfo/rails) – Elliott C. removed\_email\_address@domain.invalid removed\_email\_address@domain.invalid ---
**Author:** ![frebe73](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/f/5daacb/32.png) [@frebe73](https://www.ruby-forum.com/u/frebe73)\ **Post date:** [July 23, 2006, 4:09am UTC](https://www.ruby-forum.com/t/single-id-column/99398/4 "2006-07-23T04:09:12Z")
You can use migrations to add an auto-incrementing id column to your existing tables. ``` - dan ``` – Dan K. [mailto:removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) [http://www.dankohn.com/](http://www.dankohn.com/)tel:+1-415-233-1000 ---
**Author:** ![frebe73](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/f/5daacb/32.png) [@frebe73](https://www.ruby-forum.com/u/frebe73)\ **Post date:** [July 23, 2006, 10:48am UTC](https://www.ruby-forum.com/t/single-id-column/99398/5 "2006-07-23T10:48:21Z")
> Is it true that Rails needs a single column to uniquely identify a row > in table? If yes, doesn’t this limit the database design and disables > the use of Rails for legacy databases? Try [http://compositekeys.rubyforge.org](http://compositekeys.rubyforge.org) - its alpha code, but play with it and tell me where the holes are so I can fix them for you. Cheers Nic ---
**Author:** ![frebe73](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/f/5daacb/32.png) [@frebe73](https://www.ruby-forum.com/u/frebe73)\ **Post date:** [July 23, 2006, 5:16am UTC](https://www.ruby-forum.com/t/single-id-column/99398/6 "2006-07-23T05:16:42Z")
On 7/23/06, Elliott C. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > It is true except for the habm join tables those do not need them > since they are never instanciated as AR objects.(at least thats what > my memory is telling me.) Except that habtm and join tables have been all but outlawed in the RoR world. –