Question

I’m trying to create a model representing graph node.

class Node(models.model):
    ins = models.ManyToManyField("self", null=True, blank=True)
    outs = models.ManyToManyField("self", null=True, blank=True)

Now let’s say we have nodes a and b. If I add a as in for b django (because of ‘symmetrical’ attribute on) will add b as in for a.

I have no idea how to do it the way I set a -> b it’s automatically set b <- a.

I thought about making some middle-class for relationship but don’t really see how it would work. Literally how to write it.

I’d see it something like that:

class Node(models.Model):
    ins = models.ManyToManyField("self", null=True, blank=True, through="Edge")
    outs = models.ManyToManyField("self", null=True, blank=True, through="Edge")

class Edge(models.Model):
    node1_ins = models.ForeignKey(Node)
    node2_outs = models.ForeignKey(Node)

But of course this one doesn’t work at all.

Any idea how to solve that one?

Thanks in advance,
Greg

Answer 1

You might want to try something like this:

from django.db import models

class Node(models.Model):
    outs = models.ManyToManyField("self", null=True, blank=True, 
        symmetrical=False, related_name="ins", through='Edge')

class Edge(models.Model):
    from_node = models.ForeignKey(Node, related_name='edges_down')
    to_node = models.ForeignKey(Node, related_name='edges_up')

I found that managing multiple m2m fields would require multiple “through” tables, which just becomes messy.

n1 = Node.objects.create()
n2 = Node.objects.create()
edge = Edge.objects.create(from_node=n1, to_node=n2)

print n1.outs.all()
#[<Node: Node object>]
print n1.ins.all()
# []

print n2.outs.all()
# []
print n2.ins.all()
# [<Node: Node object>]

With this pattern you would set from-to relationships. The outs would be explicit, and the ins would be a relation. Maybe this will work?

The edges_up and edges_down relationships on the Node objects also let you find the relationship edge.

n1.edges_down.all()
# [<Edge: Edge object>]

n2.edges_up.all()
# [<Edge: Edge object>]

The names here might be unclear. I kinda like the concept of “upstream” and “downstream”