# Validates\_uniqueness\_of across multiple rows?

**URL:** <https://www.ruby-forum.com/t/validates-uniqueness-of-across-multiple-rows/177796>\
**Category:** Rails\
**Created:** [October 23, 2009, 9:15pm UTC](https://www.ruby-forum.com/t/validates-uniqueness-of-across-multiple-rows/177796 "2009-10-23T21:15:43Z")\
**Posts on this page:** 3\
**Page:** 1

<div class="post-metadata">

**Author:** ![raji](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/eb8c5e/32.png) [@raji](https://www.ruby-forum.com/u/raji)\
**Post date:** [October 23, 2009, 9:15pm UTC](https://www.ruby-forum.com/t/validates-uniqueness-of-across-multiple-rows/177796/1 "2009-10-23T21:15:43Z")

</div>

Ok, that’s not the best subject line, but I couldn’t think of a  
concise way to describe what I’m trying to do.

I’ve got two models. Grouping simply contains an id and a few other  
attributes (e.g. rating). A grouping can contain many elements which  
are stored the Element model.

A set of elements is tied together by a grouping\_id. An element can  
be one of several types of objects. I’m doing this with polymorphic  
associations.

I need a way to ensure that a grouping is unique. That is, there  
should not be two grouping\_ids containing the exact same elements.

The closest analogy I can think of is a composite primary key.  
Except, this is across multiple rows (instead of columns) and uses  
polymorphic association types and ids.

Can this be easily done with Rails and ActiveRecord?

Thanks,  
Raj

---

<div class="post-metadata">

**Author:** ![raji](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/eb8c5e/32.png) [@raji](https://www.ruby-forum.com/u/raji)\
**Post date:** [October 23, 2009, 10:20pm UTC](https://www.ruby-forum.com/t/validates-uniqueness-of-across-multiple-rows/177796/2 "2009-10-23T22:20:59Z")

</div>

raji wrote:  
[…]

> I need a way to ensure that a grouping is unique. That is, there  
> should not be two grouping\_ids containing the exact same elements.
> 
> The closest analogy I can think of is a composite primary key.  
> Except, this is across multiple rows (instead of columns) and uses  
> polymorphic association types and ids.
> 
> Can this be easily done with Rails and ActiveRecord?

Well, you could write a custom validator, something like (untested):  
class Grouping \< AR::B  
validate :must\_have\_unique\_elements

def must\_have\_unique\_elements  
taken = (elements.first.groupings.find :element\_ids =\>  
element\_ids).empty?  
# There’s gotta be a more efficient way to write that query.

```
errors.add_to_base("That combination of elements already exists!") 

```

if taken  
end  
end

I don’t think there’s anything off the shelf to make this easier, but I  
might be wrong…

> Thanks,  
> Raj

## Best,

Marnen Laibow-Koser  
[http://www.marnen.org](http://www.marnen.org)  
removed\_email\_address@domain.invalid

---

<div class="post-metadata">

**Author:** ![raji](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/eb8c5e/32.png) [@raji](https://www.ruby-forum.com/u/raji)\
**Post date:** [October 24, 2009, 8:12pm UTC](https://www.ruby-forum.com/t/validates-uniqueness-of-across-multiple-rows/177796/3 "2009-10-24T20:12:59Z")

</div>

This isn’t something that’s going to be easy in any ORM, but here’s an  
idea.

What about caching a list of the members of a Grouping in an  
attribute? You’d need to include the type obviously, so it would look  
something like this:

‘bar:1,bar:5,foo:2,frob:14’

You’d need to update this field when adding an element, and then  
validates\_uniqueness\_of and/or unique key it in the DB.

Also note that you’ll want to sort it, otherwise you’ll run into  
problems.

Dealing with users concurrently adding things to Groupings is a whole  
'nother bucket of hurt, which will be trickier to deal with.

–Matt J.