Django foreign key backward relation does not work - django

Question

I have a class B that contains a ForeignKey relation to class A. When I instantiate B I can access field 'a' but the reverse relation (which should be created automatically) raises an error. Concretely, I have the following class definitions:
from django.db.models import Model, CharField, DateField, ForeignKey
from django.urls import reverse
class Patient(Model):
GENDER = (
('M', 'Male'),
('F', 'Female'),
('U', 'Unknown'),
)
last_name = CharField(max_length=128, null=False)
first_name = CharField(max_length=128, null=False, default='')
gender = CharField(max_length=1, choices=GENDER, null=False)
dob = DateField(null=False)
def get_absolute_url(self):
return reverse('patient_detail', args=[str(self.id)])
def __str__(self):
return '{}, {} ({}, {})'.format(self.last_name, self.first_name, self.gender, self.dob)
class AttributeSet(Model):
name = CharField(max_length=128, null=False)
description = CharField(max_length=256, blank=True, default='')
def get_absolute_url(self):
return reverse('attribute_set_detail', args=[str(self.id)])
def __str__(self):
return self.name
class AttributeSetInstance(Model):
patient = ForeignKey('Patient', null=False) # Automatic 'attribute_set_instance_set' backward relation?
attribute_set = ForeignKey('AttributeSet', null=False)
def get_absolute_url(self):
return reverse('attribute_set_instance_detail', args=[str(self.id)])
def __str__(self):
return self.attribute_set.name
When I try to create a new AttributeSetInstance with a Patient and AttributeSet argument I can access the patient and attribute_set fields, but not vice versa. Like so:
Python 2.7.13 (v2.7.13:a06454b1afa1, Dec 17 2016, 12:39:47)
[GCC 4.2.1 (Apple Inc. build 5666) (dot 3)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
(InteractiveConsole)
>>> from app.models import Patient, AttributeSet, AttributeSetInstance
>>> p = Patient(last_name='Doe', first_name='John', gender='M', dob='1973-07-16')
>>> p
<Patient: Doe, John (M, 1973-07-16)>
>>> a = AttributeSet(name='Set1')
>>> a
<AttributeSet: Set1>
>>> i = AttributeSetInstance(patient=p, attribute_set=a)
>>> i
<AttributeSetInstance: Set1>
>>> i.patient
<Patient: Doe, John (M, 1973-07-16)>
>>> i.attribute_set
<AttributeSet: Set1>
>>> p.attribute_set_instance_set
Traceback (most recent call last):
File "<console>", line 1, in <module>
AttributeError: 'Patient' object has no attribute 'attribute_set_instance_set'
>>>
So, i.patient works, but p.attribute_set_instance_set does not. As far as I could figure out, the default manager that is assigned to ForeignKey relationships appends a '_set' postfix to the backward relation.
Any idea what I'm doing wrong? It would be of great help.
It's probably something silly...
Ralph

Answer 1

You've added underscores where there shouldn't be any. The default related name is the lower case name of the model plus _set, so for AttributeSetInstance it is attributesetinstance_set.
If you want to use your underscore version, you can set it as the explicit related_name argument to the ForeignKey field.