So I have a Django model which is a foreign key for several other models. I want to add a method to my model and see if my instance of the model actually has any related objects. Two questions:
-
I’d like to do this with one query, but I’m unsure how to use
prefetch_relatedwithself. How do I do this? -
Is there a way to just return
TrueorFalseif a related model exists? For example, should I just use first? (I seem to recall there being a “correct” way to do this, but it’s been a while, and I may be remembering flask.)
Code example:
from django.db import models
class Person(models.Model):
# <fields>
def has_device(self):
# ***prefetch related phones and laptops***
if self.phone_set.first() or self.laptop_set.first():
return True
return False
class Laptop(models.Model):
owner = models.ForeignKey(Person)
# <fields>
class Phone(models.Model):
owner = models.ForeignKey(Person)
# <fields>
Answers to your questions in reverse order:
- The canonical way to do this is to use the
.exists()query:
return self.phone_set.exists() or self.laptop_set.exists()
- There is no real point trying to do this in one query, as they are different tables. That being said, you can try the
prefetch_related_objectshelper:
from django.db.models import prefetch_related_objects
...
def has_device(self):
prefetch_related_objects([self], "phone_set", "laptop_set")
return self.phone_set.exists() or self.laptop_set.exists()