rubyonrailsin

DGerton wrote:
> This is question deals with the common example where Category has_many
> Products. (We'll use Species instead of Products here.)
>
> Assume Category is a nested class. Let's use this as the example:
>
> [Categories]
> Animals (id 1, parent_id=nil)
> -Mammals (id 2, parent_id=1)
> --Rodents (id 3, parent_id=2)
> -Birds (id 4, parent_id=1)
>
> [Species]
> Dog (category_id 2)
> Cat (category_id 2)
> Mouse (category_id 3)
> Rabbit (category_id 3)
> Duck (category_id 4)
>
> I want to be able to perform a select at Animals and get all of the
> Species below it, and also be able to select from Birds and just get
> Duck. It's also necessary to sort the resultant collection.
>
> This seems like a common problem and I'm overlooking a simple
> solution. Right now, though, I'm thinking I need to build up a query
> with a lot of ORs in the WHERE clause (e.g. to get mammals and the
> children I'd need :condition => "category_id = 2 OR category_id =
> 3"). This doesn't feel very rails-like.
>
> I'm not opposed to restructuring the database at this point, if that
> makes solving this problem easier.

You're using an adjacency-list tree model right now, where each node
only stores its parent ID. This has the problem you've discovered: you
can't get all descendants to arbitrary depths with one query.

The solution is to use a nested-set or nested-interval model. There are
several Rails plugins for this; awesome_nested_set is the one that I've
used most.

>
> Thanks in advance.
> Dave

Best
--
Marnen Laibow-Koser
http://www.marnen.org
marnen@marnen.org
--
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