Modernized custom manager example
Since this example was added 15 years ago in the ORM has gained the ability to do the `COUNT(*)` related query, so do it with the ORM to avoid misleading users that raw SQL is only supported from manager methods.
@@ -55,47 +55,34 @@ functionality to your models. (For "row-level" functionality -- i.e., functions
that act on a single instance of a model object -- use :ref:`Model methods
<model-methods>`, not custom ``Manager`` methods.)
-A custom ``Manager`` method can return anything you want. It doesn't have to
-return a ``QuerySet``.
-
-For example, this custom ``Manager`` offers a method ``with_counts()``, which
-returns a list of all ``OpinionPoll`` objects, each with an extra
-``num_responses`` attribute that is the result of an aggregate query::
+For example, this custom ``Manager`` adds a method ``with_counts()``::
from django.db import models
+ from django.db.models.functions import Coalesce
class PollManager(models.Manager):
def with_counts(self):
- from django.db import connection
- with connection.cursor() as cursor:
- cursor.execute("""
- SELECT p.id, p.question, p.poll_date, COUNT(*)
- FROM polls_opinionpoll p, polls_response r
- WHERE p.id = r.poll_id
- GROUP BY p.id, p.question, p.poll_date
- ORDER BY p.poll_date DESC""")
- result_list = []
- for row in cursor.fetchall():
- p = self.model(id=row[0], question=row[1], poll_date=row[2])
- p.num_responses = row[3]
- result_list.append(p)
- return result_list
+ return self.annotate(
+ num_responses=Coalesce(models.Count("response"), 0)
+ )
class OpinionPoll(models.Model):
question = models.CharField(max_length=200)
- poll_date = models.DateField()
objects = PollManager()
class Response(models.Model):
poll = models.ForeignKey(OpinionPoll, on_delete=models.CASCADE)
- person_name = models.CharField(max_length=50)
- response = models.TextField()
+ # ...
-With this example, you'd use ``OpinionPoll.objects.with_counts()`` to return
-that list of ``OpinionPoll`` objects with ``num_responses`` attributes.
+With this example, you'd use ``OpinionPoll.objects.with_counts()`` to get a
+``QuerySet`` of ``OpinionPoll`` objects with the extra ``num_responses``
+attribute attached.
+
+A custom ``Manager`` method can return anything you want. It doesn't have to
+return a ``QuerySet``.
-Another thing to note about this example is that ``Manager`` methods can
-access ``self.model`` to get the model class to which they're attached.
+Another thing to note is that ``Manager`` methods can access ``self.model`` to
+get the model class to which they're attached.
Modifying a manager's initial ``QuerySet``
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