How to select related in django model so it wont generate a lot of subqueries

Question

I have lot of models that are referring to each other in some way, for example:

Port is referring to City, which in its turn refers to Country. Then in django admin I want to show in list_display Ports country:

class Country(models.Model):
    title = models.CharField()


class City(models.Model):
    title = models.CharField()

    country = models.ForeignKey(Country)


class Port(models.Model):
    city = models.ForeignKey(City)

    def __str__(self):
        return self.city.county.title

So basically for each port django is generating more queries. I assume that select_related would help me somehow in this case, but how to use it properly within the model?

Answer 1

You can create a PortManager class and override get_queryset method and reference it in the model:

class PortManager(models.Manager):
    def get_queryset(self):
        return super(PortManager, self).get_queryset().select_related()


class Port(models.Model):
    city = models.ForeignKey(City)
    objects = PortManager()

    def __str__(self):
        return self.city.country.title

Answer 2

Based on Ivan's answer, I came up with the following structure for multiple models related with "one to many" and "many to many" relations using select_related and prefetch_related accordingly.

In the provided example only select_related is required.

class Country(models.Model):
    title = models.CharField()

    def __str__(self):
        return '{0}'.format(self.title)


class CityManager(models.Manager):
    def get_queryset(self):
        return super().get_queryset().select_related('country')


class City(models.Model):
    title = models.CharField()
    country = models.ForeignKey(Country)
    objects = CityManager()

    def __str__(self):
        return '{0}-{1}'.format(
            self.title,
            self.country.__str__(),
        )


class PortManager(models.Manager):
    def get_queryset(self):
        return super().get_queryset().select_related('city')


class Port(models.Model):
    city = models.ForeignKey(City)
    objects = PortManager()

    def __str__(self):
        return self.city.__str__()