Access RelatedManager object in django

Question

I have the following model :

class Travel(models.Model):
    purpose = models.CharField(max_length=250, null=True)
    amount = models.IntegerField()
class TravelSection(models.Model):
    travel = models.ForeignKey(Travel, related_name='sections', on_delete=models.CASCADE)
    isRoundTrip = models.BooleanField(default=True)
    distance = models.FloatField()
    transportation = models.CharField(max_length=250)

I create object without using the database :

mytravel = Travel(
  purpose='teaching',
  amount=1
)
mysection = TravelSection(
  travel=mytravel,
  isRoundTrip=True,
  distance=500,
  transportation='train'
)

When I try to access mytravel.sections, I can't access to the field distance and other fields. How can I do this without having to save in the database? I don't want to create objects in the database, because I'm trying to make some preprocessing.

You are trying to access it using `mytravel.sections.distance` ? — Ersain, Jun 10 '22 at 09:04
Actually, `RelatedManager` is similar to standard `Manager` and uses database to access related objects. When you have just this two objects in python, `mytravel` does not know anything about `mysection`. It is working with database `JOIN`. How do you expect such reverse relationship to exist in pure python? Only if all foreign keys are handled in `__init__` and added reversely, but it would be very inefficient. — STerliakov, Jun 10 '22 at 09:04

Kolton Noreen · Answer 1 · 2022-06-10T09:14:00.443

Think of the Manager as a query builder. Managers do not have access to individual values for any particular instance, you must use the RelatedManager's methods to get a QuerySet such as mytravel.sections.all(). From there you will have a QuerySet that you can iterate through to get each mysection and access distance from there.

RelatedManagers ARE managers, they just have extra functionality. Now this is confusing because there also QuerySets which are similar to Managers except they have a query already stored.

Access RelatedManager object in django

1 Answers1