Django query: Item in model who is not in other model

Question

Noob Django ORM question here:

I have a model similar to this:

class Company(models.Model): id = models.PositiveIntegerField(null=False, blank=False, primary_key=True) name = models.CharField(max_length=255, null=True, blank=True, default="") class Audited(models.Model): company = models.ForeignKey( Company, null=False, blank=False, on_delete=models.CASCADE, related_name="logs" ) city = models.CharField(null=False, blank=False, max_length=255)

I have been scratching my head with this problem and I can not figure out how to solve it.

Given a city, I need the company that do not have been audited on that city, so basically the Company who does not have a record on Audited with that city.

So for example, if the city is New York, and there is no record in Audited with a company like Oracle and city as New York, I receive Oracle.

It is very confusing to me because the ORM appears to have been designed for positive relations, like "the item from table A who has a record on Table B", and this is precisely the opposite. Any clue of how can I do this?

Yes, there can be multiple companies, but I always will need the first one, because after that I will add the row to Audited (for example, Oracle-New York), so the next time I asked for the next company to audit on New York, Oracle will be ignored. After the table is full (all the companies have records in New York, for example), I have to use other logic (the company audited on that city with the oldest date) — John Smith
– John Smith, Commented Sep 10, 2020 at 0:52
You should remove the id field from Company model as django creates one automatically. — Onyilimba
– Onyilimba, Commented Sep 10, 2020 at 2:21

willeM_ Van Onsem · Accepted Answer · 2020-09-10 00:50:47Z

You can filter with .exclude(…) [Django-doc]:

Company.objects.exclude(logs__city=my_city)

So you will retain Companys for which no Audited exists for the given my_city.

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