Hi all,
I've run into many situations during my time using Django where I've wanted
to be able to express relations based on some other criteria than foreign
key equality. A few examples:
- descendants or children of a node in a tree structure
- saved search terms to search results
- a model containing a date range to timestamped items falling within that
date range
Currently to do this kind of thing, you might write a getter which returns
a queryset - think for example mptt's get_descendants(). But you don't get
any of the nice things a real relation field gives you - you can't use that
relationship in filters, you can't select/prefetch_related() or values(),
there's no reverse relationship, etc.
I've written a Relationship field[0] that lets you define relations in
terms of arbitrary Q filters containing objects of a new type, L. An L is
like an F, but represents a field on the "local" or "left" side of the
relation, where the Q is filtering against the remote "to" side of the
relation. For example, in a materialised path tree, this is how you might
express descendants:
class Node(models.Model):
path = models.TextField()
descendants = Relationship(
"self",
Q(path__startswith=L('path'), pk__ne=L('pk')),
multiple=True,
reverse_multiple=True,
related_name='ascendants',
)
Now you can use the descendants field like any other many-to-many field in
all the places I mentioned above, but the relationship is based purely on
prefix-matching on the path field. You also get an ascendants field on
Node, which represents the path back to the root and can be used in the
same way.
I think this could make a nice new feature for Django. It would give a
usability boost to anyone using MPTT or treebeard, for example. It works OK
as a third-party library, but the current implementation relies heavily on
undocumented ORM internals, and there are a few features I'd like to
implement that are impractical without making some ORM changes.
Thoughts/feedback/questions welcome!
Thanks,
Alex
[0] https://github.com/alexhill/django-relativity
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