On 24 fév, 18:08, Peter Reimer <pet.rei...@googlemail.com> wrote:
> Hi folks,
>
> I'm looking for a solution/tip for the following problem: I need to
> store hierarchical data, but, with two different kinds of objects
> (groups and items).
That's something the relational model (at least in it's SQL
implementation) is not very good at.
> I googled around and found the often suggested
> mptt package.
The mptt pattern is mostly useful for deep hierarchies that are often
queried for a whole branch at once and rarely updated.
If you have a not-that-deep tree with frequent updates and don't need
to retrieve whole branches at once, the adjacency list or materialized
path patterns (or a combination of both) might yield better results.
>
> One idea is, to build the hierarchical structure with one model and
> mptt and in it, I define two ForeignKeys to the concrete data objects
> I want to store (ForeignKey(Group) and Foreignkey(Item)). But this
> sounds a bit strange to me. I think there should be a much smarter
> way.
>From the OOD perspective, this looks like a canonical use case for the
composite design pattern. Applied to Django's models and using the
adjacency list pattern, you'd have a Node base class with Groups and
Items subclasses (Q&D example, may contains obvious stupid errors):
class Node(models.Model):
parent = models.ForeignKey(
"self",
blank=True,
null=True,
related_name="children"
)
def save(self, *args, **kw):
if self.parent and type(self.parent) != Group:
raise ValueError("only Group nodes can be parents")
super(Node, self).save(*args, **kw)
class Group(Node):
# code here
class Item(Node):
# code here
Don't know if and how it would solve your problem, but this might get
you started one way or another... Else, well, sorry but that's all I
have :-/
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