# HABTM \<\< Behavior

**URL:** <https://www.ruby-forum.com/t/habtm-behavior/152647>\
**Category:** Rails\
**Created:** [November 6, 2008, 12:02am UTC](https://www.ruby-forum.com/t/habtm-behavior/152647 "2008-11-06T00:02:52Z")\
**Posts on this page:** 5\
**Page:** 1

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**Author:** ![casper\_the\_ghost](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/c/58956e/32.png) [@casper\_the\_ghost](https://www.ruby-forum.com/u/casper_the_ghost)\
**Post date:** [November 6, 2008, 12:02am UTC](https://www.ruby-forum.com/t/habtm-behavior/152647/1 "2008-11-06T00:02:52Z")

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Lets say you have a relationship like post has and belongs to many  
categories. If I were to do something like the following:

category \<\< post  
category \<\< post  
category \<\< post

I’ve basically assigned the post to the category three times. I’m  
noticing that in my join table there are now three join records?  
Shouldn’t it only have one?

This causes problems when you do category.posts and you get the same  
post three times. I’m just wondering if I am missing something here?

Thanks,  
Tom

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**Author:** ![casper\_the\_ghost](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/c/58956e/32.png) [@casper\_the\_ghost](https://www.ruby-forum.com/u/casper_the_ghost)\
**Post date:** [November 6, 2008, 11:22am UTC](https://www.ruby-forum.com/t/habtm-behavior/152647/2 "2008-11-06T11:22:21Z")

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On 5 Nov 2008, at 23:01, TomRossi7 wrote:

> Shouldn’t it only have one?  
> If that’s what you require, set the :uniq option on the association  
> (and create a unique index on that table to enforce that if you are so  
> enclined)

Fred

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<div class="post-metadata">

**Author:** ![casper\_the\_ghost](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/c/58956e/32.png) [@casper\_the\_ghost](https://www.ruby-forum.com/u/casper_the_ghost)\
**Post date:** [November 6, 2008, 3:01pm UTC](https://www.ruby-forum.com/t/habtm-behavior/152647/3 "2008-11-06T15:01:58Z")

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Fred,

Thanks for the reply! The :uniq is good work around, but multiple  
entries are still added to the database. The unique index will  
actually cause ActiveRecord to throw an error since it is trying to  
create a duplicate record.

This seems like a bug to me that it creates duplicate records? I can  
code around it, but wanted to make sure I wasn’t missing something.

Thanks,  
Tom

On Nov 6, 5:21 am, Frederick C. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid)

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<div class="post-metadata">

**Author:** ![casper\_the\_ghost](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/c/58956e/32.png) [@casper\_the\_ghost](https://www.ruby-forum.com/u/casper_the_ghost)\
**Post date:** [November 6, 2008, 3:38pm UTC](https://www.ruby-forum.com/t/habtm-behavior/152647/4 "2008-11-06T15:38:24Z")

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On 6 Nov 2008, at 14:00, TomRossi7 wrote:

> 

I don’t think it’s necessarily a bug. Depend on how you are using it  
duplicate records in an association might be allowed, or might not,  
it’s not the frameworks job to say ‘collections shall have no  
duplicates’. I’m slightly surprised that uniq only affects reading  
from the collection, I suppose the thing is that normally categories  
\<\< post does not require loading the collection whereas if it checked  
if categories.include?(post) that would force the collection to be  
loaded (at least it would have previously - I seem to recall the  
include? no longer causes the collection to be loaded)

Fred

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<div class="post-metadata">

**Author:** ![casper\_the\_ghost](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/c/58956e/32.png) [@casper\_the\_ghost](https://www.ruby-forum.com/u/casper_the_ghost)\
**Post date:** [November 6, 2008, 3:45pm UTC](https://www.ruby-forum.com/t/habtm-behavior/152647/5 "2008-11-06T15:45:26Z")

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Fred,

Yeah, you are totally tracking with what I am saying. I think it is a  
bug though because in a 1 to many relationship, or in a many to many  
relationship, the same join shouldn’t exist more than once? Is there  
a scenario where it even makes sense?

The :uniq option just has ruby remove duplicates from the array  
returned (e.g. category.posts.uniq). The categories.include?(post)  
code around is the one I am going with, its just not as elegant as I  
was hoping for. Now I just need to do a comparison to see if the  
child has already been assigned to the parent before adding it.

Thanks!  
Tom

On Nov 6, 9:37 am, Frederick C. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid)