I'm using a small tree/graph package (django_dag) that has that give my model a Many-to-many children field which refers to itself. The basic structure can be shown as the following models
#models
class Foo(FooBase):
class Meta:
abstract = True
children = models.ManyToManyField('self', symmetrical = False,
through = Bar)
class Bar():
parent = models.ForeignKey(Foo)
child = models.ForeignKey(Foo)
All is fine with the models and all the functionality of the package.
FooBase adds a variety of functions to the model, including a way of recursively finding all children of a Foo and the children's children and so forth.
My concern is with the following function within FooBase:
def descendants_tree(self):
tree = {}
for f in self.children.all():
tree[f] = f.descendants_tree()
return tree
It outputs something like {Foo1:{}, Foo2: {Child_of_Foo2: {Child_of_Child_of_Foo2:{}}}} where the progeny are in a nested dictionary.
The alert reader may notice that this method calls a new query for each child. While these db hits are pretty quick, they can add up quickly when there might be 50+ children. And eventually, there will be tens of thousands of db entries. Right now, each query averages 0.6 msec with a row count of almost 2000.
Is there a more efficient way of doing this nested query?
In my mind, doing a select_related().all() beforehand would get it down to one query but that smells like trouble in the future. At what point is one large query better or worse than many small ones?
---Edit---
Here's what I'm trying to test the select_related().all() option with, but it's still hitting every iteration:
all_foo = Foo.objects.select_related('children').all()
def loop(baz):
tree = {}
for f in all_foo.get(id = baz).children.all()
tree[f] = loop(f)
return tree
I assume the children.all() is causing the hit. Is there another way to get all of Many-to-Many children without using the callable attribute?
