expanding the SQL query inside managers in Django models? - mysql

Question

Here is the code from django docs that explains the use of managers.
class PollManager(models.Manager):
def with_counts(self):
from django.db import connection
cursor = connection.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
class OpinionPoll(models.Model):
question = models.CharField(max_length=200)
poll_date = models.DateField()
objects = PollManager()
class Response(models.Model):
poll = models.ForeignKey(OpinionPoll)
person_name = models.CharField(max_length=50)
response = models.TextField()
I have two questions based on this code:
1) where is r.poll_id coming from? I understand Response has foreignKey relationship to OpinionPoll. In order to JOIN OpinionPoll table with Response table, I need to join on their id.
HOwever to access the poll id in Response, I would do r.poll.id.
Is the syntax, r.poll_id, a MySQL syntax.
why GROUP BY p.id, p.question, p.poll_date? why GROUP BY p.id alone is not sufficient?
2) Is it possible to turn the above raw SQL query into a django ORM query?If so how would that look like?
I am not a SQL guy. so bear with me, if this sounds stupid
EDIT:
If I want to create OpinionPoll and Response tables outside of Django, how will SQL statment for create look like?
In the Django shell, when I run
python manage.py sqlall appname
I get the following:
BEGIN;
CREATE TABLE "myapp_opinionpoll" (
"id" integer NOT NULL PRIMARY KEY,
"question" varchar(200) NOT NULL,
"poll_date" date NOT NULL
)
;
CREATE TABLE "myapp_response" (
"id" integer NOT NULL PRIMARY KEY,
"poll_id" integer NOT NULL REFERENCES "myapp_opinionpoll" ("id"),
"person_name" varchar(50) NOT NULL,
"response" text NOT NULL
)
;
CREATE INDEX "larb_response_70f78e6b" ON "myapp_response" ("poll_id");
COMMIT;
I see something like REFERENCES "myapp_opinionpoll" and CREATE INDEXabove. I am not sure
if this is how in SQL it is done?

Answer 1

[1] Django model will create foreign keys like fieldname_id as the field in mysql. So you see the field poll = models.ForeignKey(OpinionPoll) creates this field.
About GROUP BY, because these fields are exactly what selected, except for the aggregate function, grouping them exactly can make them distinct.
[2] Try this, I didn't debug, but may helps:
from django.db.models import Count
OpinionPoll.objects.annotate(num_responses=Count('response'))
For more about aggregation, see the docs: https://docs.djangoproject.com/en/1.6/topics/db/aggregation/