Django: prefetch_related results ordered by a field of an intermediary table

Question

How can I prefetch_related objects in Django and order them by a field in an intermediary table? Here's the models I'm working with: ``` class Node(models.Model): name = models.CharField(max_length=255) edges = models.ManyToManyField('self', through='Edge', symmetrical=False) class Edge(models.Model): from_node = models.ForeignKey(Node, related_name='from_node') to_node = models.ForeignKey(Node, related_name='to_node') weight = models.FloatField(default=0) ``` Given a node, I'd like to prefetch all of the related nodes, ordered by weight. When I use this query: ``` n = Node.objects.prefetch_related('to_node').order_by('edge__weight').get(name='x') ``` the order_by has no effect. Edit: My best answer so far ``` n = Node.objects.get(name='x') edges = Edge.objects.filter(from_node=n).prefetch_related('to_node').order_by('weight') ``` Then instead of iterating `n.edges` (as I'd prefer), I iterate `edges.to_node`

Accepted Answer

Just a conceptual idea (written from memory).

The problem is, that the `order_by` refers to the Node model.

However, there is a way to

```
Node.objects.get(name='x').edges.extra(select={'weight':'%s.weight' % Edge._meta.db_table}).order_by('weight')
```

This will force the ORM to:

- Add 'weight' field, which would normally be omitted.

- Order the results by it.

Number of queries should be the same as if the `prefetch_query` worked, one to get the Node, second to get the related nodes.

Unfortunately this is not a very 'clean' solution, as we need to use `_meta`.

Django: prefetch_related results ordered by a field of an intermediary table

Solution

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