There is this line in the Django tutorial, Writing your first Django app, part 1:
p.choice_set.create(choice='Not much', votes=0)
choice_set called into existence and what is it?
I suppose the
choice part is the lowercase version of the model
Choice used in the tutorial, but what is
choice_set? Can you elaborate?
You created a foreign key on
Choice which relates each one to a
Choice explicitly has a
question field, which you declared in the model.
Django’s ORM follows the relationship backwards from
Question too, automatically generating a field on each instance called
Foo is the model with a
ForeignKey field to that model.
choice_set is a
RelatedManager which can create querysets of
Choice objects which relate to the
Question instance, e.g.
If you don’t like the
foo_set naming which Django chooses automatically, or if you have more than one foreign key to the same model and need to distinguish them, you can choose your own overriding name using the
related_name argument to
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)
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
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:
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.