Django database: how to filter objects based on other object's field?

Question

In the code below, how do I filter capital_listings so to return listings only for capital cities? Also, is it possible to get rid of the intermediate capitals_names list?

capitals = City.objects.filter(status='capital')
capitals_names = [capital.name for capital in capitals]
capital_listings = Listing.objects.filter #???

Models:

class Listing(models.Model):
    city = models.CharField(max_length = 30, default = 'placeholder')
    date_added = models.DateTimeField()

    def __str__(self):
        return self.name

class City(models.Model):
    name = models.CharField(max_length = 30, default = 'placeholder')
    status = models.CharField(max_length = 30, default = 'placeholder')

    def __str__(self):
        return self.name

docs.djangoproject.com/en/2.1/topics/db/models/#relationships — Håken Lid
– Håken Lid, Commented Aug 12, 2018 at 12:52
@JonClements: No particular reason. How would I apply it in this case? — barciewicz
– barciewicz, Commented Aug 12, 2018 at 13:22
The Django tutorial is fairly good - if you have a look at the link that @HåkenLid posted above and the examples in it - that'll explain everything you need for it (and probably more that'll come in handy) — Jon Clements
– Jon Clements, Commented Aug 12, 2018 at 13:23

Gevorg Davoian · Accepted Answer · 2018-08-12 13:09:31Z

2

capital_listings = Listing.objects.filter(city__in=capital_names)

answered Aug 12, 2018 at 13:09

Gevorg Davoian

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Håken Lid · Accepted Answer · 2018-08-12 14:10:07Z

0

If you change Listings.city to a foreignkey relationship, you can do your query in a single step.

class City(models.Model):
    name = models.CharField(max_length = 30, default = 'placeholder')
    status = models.CharField(max_length = 30, default = 'placeholder')

class Listing(models.Model):
    city = models.ForeignKey(City, on_delete=models.CASCADE)


capital_listings = Listing.objects.filter(city__status='capital')

This is explained in the docs:

https://docs.djangoproject.com/en/2.1/topics/db/queries/#lookups-that-span-relationships

answered Aug 12, 2018 at 14:10

Håken Lid

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