# Multiple keys table **URL:** \ **Category:** Rails\ **Created:** [February 28, 2006, 3:26pm UTC](https://www.ruby-forum.com/t/multiple-keys-table/74739 "2006-02-28T15:26:19Z")\ **Posts on this page:** 5\ **Page:** 1
**Author:** ![shiko](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/e47774/32.png) [@shiko](https://www.ruby-forum.com/u/shiko)\ **Post date:** [February 28, 2006, 3:26pm UTC](https://www.ruby-forum.com/t/multiple-keys-table/74739/1 "2006-02-28T15:26:19Z")
probablly a newbie question: Rails does not support a table with multiple keys ??? it seems that the ActiveRecord set\_primary\_key method can only set the column name… (hope i’m wrong there) example: table A - P.K id, string name table B - P.K id, string name table C - P.K a\_id and b\_id, both are also foreign keys. Thanks, Amir. ---
**Author:** ![shiko](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/e47774/32.png) [@shiko](https://www.ruby-forum.com/u/shiko)\ **Post date:** [February 28, 2006, 4:11pm UTC](https://www.ruby-forum.com/t/multiple-keys-table/74739/2 "2006-02-28T16:11:28Z")
amir lidor wrote: > probablly a newbie question: > Rails does not support a table with multiple keys ??? > it seems that the ActiveRecord set\_primary\_key method can only set the > column name… (hope i’m wrong there) Sorry to be the carrier of bad news, but you are correct as far as I know. ---
**Author:** ![shiko](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/e47774/32.png) [@shiko](https://www.ruby-forum.com/u/shiko)\ **Post date:** [February 28, 2006, 4:19pm UTC](https://www.ruby-forum.com/t/multiple-keys-table/74739/3 "2006-02-28T16:19:32Z")
amir lidor wrote: > probablly a newbie question: > Rails does not support a table with multiple keys ??? > it seems that the ActiveRecord set\_primary\_key method can only set the > column name… (hope i’m wrong there) > > example: > table A - P.K id, string name > table B - P.K id, string name > table C - P.K a\_id and b\_id, both are also foreign keys. What is the relationship between tables A and B ? One to many, many to one, many to many ? I just wondered if your normalization was correct… \_tony ---
**Author:** ![shiko](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/e47774/32.png) [@shiko](https://www.ruby-forum.com/u/shiko)\ **Post date:** [March 1, 2006, 7:57am UTC](https://www.ruby-forum.com/t/multiple-keys-table/74739/4 "2006-03-01T07:57:29Z")
Anthony G. wrote: > amir lidor wrote: > > > probablly a newbie question: > > Rails does not support a table with multiple keys ??? > > it seems that the ActiveRecord set\_primary\_key method can only set the > > column name… (hope i’m wrong there) > > > > example: > > table A - P.K id, string name > > table B - P.K id, string name > > table C - P.K a\_id and b\_id, both are also foreign keys. > > What is the relationship between tables A and B ? > One to many, many to one, many to many ? > I just wondered if your normalization was correct… > \_tony The relation is many to many. I talked with a friend though and he explained me that it’s really bad to use multiple-keys tables, and that if i assign another primary key column, even if there will be multiple identical (value-wise of the a\_id and b\_id columns) rows, RoR will ignore them when performing Joins. so i understand what should i do, I am just left with the question (out of curiosity) about Rails supporting multiple-keys tables… Thanks, Amir. ---
**Author:** ![shiko](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/e47774/32.png) [@shiko](https://www.ruby-forum.com/u/shiko)\ **Post date:** [March 1, 2006, 2:58pm UTC](https://www.ruby-forum.com/t/multiple-keys-table/74739/5 "2006-03-01T14:58:06Z")
On Mar 1, 2006, at 1:57 AM, amir lidor wrote: > > > table B - P.K id, string name > > > column, even if there will be multiple identical (value-wise of the > > > a\_id > > > and b\_id columns) rows, RoR will ignore them when performing Joins. > > > so i understand what should i do, I am just left with the question > > > (out > > > of curiosity) about Rails supporting multiple-keys tables… > > Thanks, > Amir. It sounds like you want a composite primary key, i.e. the combination of (a\_id, b\_id) form the primary key for table C. No, this is not supported in Rails. Such practice has fallen out of favor in recent years, and some database servers do not even support it. There is still much debate, but Rails has taken the position of rejecting composite primary keys. -Derrick S.