CODEWITHSUNDEEP
pythondjangopostgresql
ingernet
ingernet

Reputation: 1524

Django/Postgres: displaying distinct ManyToMany items

Let's say for the sake of simplicity that I have three classes:

class BigThing(models.Model):
    title = models.CharField(max_length=48)
    stufflists = models.ManyToManyField(StuffList)

class StuffList(models.Model):
    title = models.CharField(max_length=48)
    stuff = models.ManyToManyField(Stuff)

class Stuff(models.Model):
   code = models.CharField(max_length=24)
   title = models.CharField(max_length=48)
   description = RedactorField(blank=True, null=True)

BigThing1 contains List1 and List3.

List1 contains Item1, Item2, Item3 in it.

List2 contains Item1, Item2, and Item4.

List3 contains only Item3.

I would like to return all of the Items that belong to BigThing1, omitting duplicates, like so: 

Item1, Item2, Item3
NOT: Item1, Item2, Item3, Item3
(and obviously not Item4, since that doesn't belong to either of the lists associated with BigThing1)

I feel like there's something involving prefetch_related or select_related, but am kind of stumped because I'm new to Django/Postgres. In the past I would have done a JOIN query, but acknowledge that those queries are kind of hacks anyway, and would like to know the best practice for this kind of query, anyway.

Upvotes: 0

Views: 59

Answers (1)

Iain Shelvington
Iain Shelvington

Reputation: 32244

ManyToMany relationships can be followed backwards (specifically the section that starts "Reverse m2m queries are supported")

Something like the following should work

Stuff.objects.filter(stufflist__bigthing=BigThing1).distinct()

Upvotes: 1

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