Two crucial questions are asked here. First: How is choice_set
called into existence. Second: What is it?
For all new developers like me, Let me describe how I made it easy for me. Let me answer the second question first. "What is it", through these 3 words? Model Instance, Objects-set related to that instance, Related_manager.
Models.py from Django tutorial:
from django.db import models
class Question(models.Model):
question_text = models.CharField(max_length=200)
pub_date = models.DateTimeField('date published')
class Choice(models.Model):
question = models.ForeignKey(Question, on_delete=models.CASCADE)
choice_text = models.CharField(max_length=200)
votes = models.IntegerField(default=0)
Instance:
q = Question.objects.get(pk=3)
# Here q is an instance of model class 'Question'.
c = Choice.objects.get(pk=32)
# Here c is an instance of model class 'Choice'.
'Model Instance' is a single 'ROW' of an entire 'TABLE' of your database
Here, the Question Model
is used as a foreign key
to the Choice Model
. Therefore, all the objects-set related to instance q can be filtered by using:
q.choice_set.all()
Therefore, choice_set
here is, all the choices related to the question, that has pk=3.
Now, the answer to the first question needs the third word Related Manager. Django documentation here:-
If a model has a ForeignKey, instances of the foreign-key model will
have access to a Manager that returns all instances of the first
model. By default, this Manager is named FOO_set, where FOO is the
source model name, lowercased. This Manager returns QuerySets, which
can be filtered and manipulated as described in the “Retrieving
objects” section above.
This word (choice_set) can be changed using the 'related_name' parameter in the Foreign_key.
question = models.ForeignKey(Question, on_delete=models.CASCADE, related_name="choices")
For backward relationtionship through foreign key:
q.choice_set.all()
# If using related_name, then it is same as
q.choices.all()
# All the choices related to the instance q.
For forward relationship:
choice_qs = Choice.objects.all()
choice_qs.filter(question=q)
# Same result as above. All the choices related to instance q.