# Many-to-many on a single table?

**URL:** <https://www.ruby-forum.com/t/many-to-many-on-a-single-table/141673>\
**Category:** Rails\
**Created:** [June 29, 2008, 1:25am UTC](https://www.ruby-forum.com/t/many-to-many-on-a-single-table/141673 "2008-06-29T01:25:15Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![robb](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/a183cd/32.png) [@robb](https://www.ruby-forum.com/u/robb)\
**Post date:** [June 29, 2008, 1:25am UTC](https://www.ruby-forum.com/t/many-to-many-on-a-single-table/141673/1 "2008-06-29T01:25:15Z")

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Hi,

I’m implementing a M-M relationship with a single table upon itself,  
and wonder if there’s anything I should look out for in rails, when  
doing this. Also, what notation I would best use in Rails (I’m  
guessing :through).

I’m doing this because I have a Category model, and each category  
might be “similar to” a number of other categories. So, one way of  
modelling this is to have a new join table with two columns which are  
both Category ids.

Thanks,  
Robb

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**Author:** ![robb](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/a183cd/32.png) [@robb](https://www.ruby-forum.com/u/robb)\
**Post date:** [June 29, 2008, 2:20am UTC](https://www.ruby-forum.com/t/many-to-many-on-a-single-table/141673/2 "2008-06-29T02:20:11Z")

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Robb wrote:

> both Category ids.
> 
> Thanks,  
> Robb
> 
> >

My guide may be able to help you out here:

> **[frozenplague.net](http://frozenplague.net/2008/04/29/self-referrential-relationships/)**
>
> This domain may be for sale!

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**Author:** ![robb](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/a183cd/32.png) [@robb](https://www.ruby-forum.com/u/robb)\
**Post date:** [June 29, 2008, 2:40am UTC](https://www.ruby-forum.com/t/many-to-many-on-a-single-table/141673/3 "2008-06-29T02:40:40Z")

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That’s excellent. Thanks!

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**Author:** ![robb](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/a183cd/32.png) [@robb](https://www.ruby-forum.com/u/robb)\
**Post date:** [June 29, 2008, 5:13pm UTC](https://www.ruby-forum.com/t/many-to-many-on-a-single-table/141673/4 "2008-06-29T17:13:41Z")

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Ryan,  
I clicked your link and found a title page but no content?  
Kathleen