# About duplication in a (HABTM) join table

**URL:** <https://www.ruby-forum.com/t/about-duplication-in-a-habtm-join-table/112777>\
**Category:** Rails\
**Created:** [August 23, 2007, 5:07am UTC](https://www.ruby-forum.com/t/about-duplication-in-a-habtm-join-table/112777 "2007-08-23T05:07:43Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![midwaltz](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/a3d4f5/32.png) [@midwaltz](https://www.ruby-forum.com/u/midwaltz)\
**Post date:** [August 23, 2007, 5:07am UTC](https://www.ruby-forum.com/t/about-duplication-in-a-habtm-join-table/112777/1 "2007-08-23T05:07:43Z")

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I’m new to rails and databases, needless to say I’m pretty confused  
here. There’s this issue I don’t understand how to resolve, heck I  
don’t even know if it’s an issue. I’ve also been searching for answers  
here, forums, irc, and nothing. Anyway I’ll try to be as clear as I  
can be. And I apologize for the many questions in one thread.

Let’s say I have two tables, girls and boys (to spicy up this topic),  
with their respective models, which in turn have a  
has\_and\_belongs\_to\_many relationship. For that relationship to work, I  
have a join table called boys\_girls, with two columns, boy\_id and  
girl\_id.

in rails, I create a boy called brad, and a girl called angelina.  
now if I do:  
brad.girls \<\< angelina  
brad.girls \<\< angelina

not only will angelina be two times in his array (if only this stuff  
could be in real life), but that relationship will appear in two rows  
on the join table.

First question: as far as database performance and size goes, is this  
a problem?

Anyway, if I add uniq =\> true to has\_and\_belongs\_to\_many on the  
models, ActiveRecord will successfully ignore this duplication when I  
reload the objects. But it will still act the same way as I said  
before: if I add duplication, it will show up in the existing array  
and it will be added to the table.

So, my second question is, how do I avoid this duplication?

I found in the agile web dev book that I can add an index to the join  
table right in the migration, and add :unique =\> true after the  
add\_index call. I have tried it, and no difference. I suppose it only  
configures the index to ignore duplicates, which would then resolve  
performance issues?

Also, I have read that validates\_uniqueness\_of accepts various columns  
with scope, but I’m not sure how to do that, and also, am I right to  
say that scope only helps to limit uniqueness to given sets of rows?  
In that case that wouldn’t help, right?

And, in case validation in the model is the way to go, where should I  
put it? In the Girl and Boy models? or should I create a model to  
represent the join table rows and do that validation there?

(Boys and girls might not exactly illustrate what I need. In my case,  
I really don’t need to add more information to the join table, so  
validation would possibly be the only reason to create a third model.)

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<div class="post-metadata">

**Author:** ![midwaltz](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/a3d4f5/32.png) [@midwaltz](https://www.ruby-forum.com/u/midwaltz)\
**Post date:** [August 24, 2007, 1:38pm UTC](https://www.ruby-forum.com/t/about-duplication-in-a-habtm-join-table/112777/2 "2007-08-24T13:38:41Z")

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I resolved the issue with the last option, creating a join model  
(let’s say I called it Bond), and a has\_many through call within both  
Girl and Boy. Then all I had to do was add this to the join model:

validates\_uniqueness\_of :boy\_id, :scope =\> :girl\_id

now everything works perfectly, and the models prevent from saving  
duplicates, returning a validation error if I try to save the same  
item twice on the same collection.

about issues of having duplicate items on a join table, in db terms, I  
talked to a much more db experienced person than me, which said that  
first it’s messy, and second it could become a problem with a very  
large user base (thousands of users with thousands of items…). So I  
guess the right way is to avoid duplication, specially when deploying  
for a large userbase, but as an internal tool for few users, it’s not  
a terrible issue.

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<div class="post-metadata">

**Author:** ![midwaltz](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/a3d4f5/32.png) [@midwaltz](https://www.ruby-forum.com/u/midwaltz)\
**Post date:** [August 24, 2007, 2:02pm UTC](https://www.ruby-forum.com/t/about-duplication-in-a-habtm-join-table/112777/3 "2007-08-24T14:02:45Z")

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Thanks for posting your solution. I was getting into habtm/through and  
I  
hadn’t seen the obvious nature using through with validations.

This could qualify as the first installment of a Rails Quiz. Want to  
start  
it?

midwaltz wrote:

> item twice on the same collection.
> 
> >

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View this message in context:  
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Sent from the RubyOnRails Users mailing list archive at [Nabble.com](http://Nabble.com).

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<div class="post-metadata">

**Author:** ![midwaltz](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/a3d4f5/32.png) [@midwaltz](https://www.ruby-forum.com/u/midwaltz)\
**Post date:** [August 25, 2007, 3:56am UTC](https://www.ruby-forum.com/t/about-duplication-in-a-habtm-join-table/112777/4 "2007-08-25T03:56:19Z")

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hi peter. a rails quiz? 🙂 go ahead, use it…

although further on I won’t be of much help, I’m might have a lot of  
questions, but no answers. I’m really new to rails and backend. Which  
is also a good reason not to take what I say for granted.

oh well. leaving tomorrow for holidays. only sand and trees. no join  
tables and validations there.