Chris Curvey ha scritto:
> I can't believe this is not a FAQ,
Yes, it is.. googling "sql tree" gives relevant results on both web and
newsgroups.
> If I have a hierarchy of categories, and my model looks like this:
>
> class Category(SQLObject):
> name = UnicodeCol(length=100)
> parent = ForeignKey("Category")
>
> How do I create a method on Category so that I can retrieve all the
> children for a Category instance?
You're working with trees of unbounded depth, by chance?
Keep in mind that an adjacency list like yours is not scalable. How many
rows are there, or will ever be?
You are free to go with the simple implementation, until you have
performance issues.
If, and when your data set grows and queries become slow, do not blame
the DBMS, because it's time to use a more appropriate data structure.
You could for instance look for "SQL For Smarties" and "trees and
hierarchies in SQL" by Joe Celko
For a summary of the treated concept, see
http://www.dbazine.com/oracle/or-articles/tropashko4
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