Django: Retrieving IDs of manyToMany fields quickly

python database django postgresql m2m
Noah Gilmore picture Noah Gilmore · Jun 30, 2013 · Viewed 14.4k times · Source

I have the following model schema in Django (with Postgres). 

class A(Models.model):
    related = models.ManyToManyField("self", null=True)

Given a QuerySet of A, I would like to return a dictionary mapping each instance of A in the QuerySet to a list of ids of its related instances as quickly as possible.

I can surely iterate through each A and query the related field, but is there a more optimal way?

Answer

Luan Fonseca picture Luan Fonseca · Jun 30, 2013

According you have Three instances. You can use the values_list method to retrieve just the results and from this result get just the ID's of their related instances. I use the pk field to be my filter because i don't know your scheme, but you can use anything, just must be a QuerySet.

>>> result = A.objects.filter(pk=1)
>>> result.values('related__id')
[{'id': 2}, {'id': 3}]
>>> result.values_list('related__id')
[(2,), (3,)]
>>> result.values_list('related__id', flat=True)
[2, 3]

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