For some people, especially those who are new to ORM, will have troubles in getting used to the way Django making queries. I was one of them, but now, it is actually helping me a lot. I don’t need to deal with query language anymore, all I need to do is trying to understand how the API works, and it’s in python code, the same with the Django itself. Django documentation somehow didn’t put clearly how to make a query with group by, so I think it is quite helpful if I post something about it here.
Let say I have this model:
number = models.CharField(max_length=255)
class Book(models.Model):
shelf = models.ForeignKey(Shelf)
title = models.CharField(max_length=255)
count = models.IntegerField(default=0)
Then I want to grouping my books by the shelf, and count the books for each shelf. The code will be like this:
And that’s it, I got the data that I wanted to retrieve. In the background, Django actually generated sql code like this:
‘SELECT “library_book”.”shelf_id”, SUM(“library_book”.”count”) AS “total”
FROM “library_book”
GROUP BY “library_book”.”shelf_id”, “library_book”.”shelf_id”
LIMIT 21′
You can also grouping from the shelf class with this code:
Which will generate something like this in the background:
‘SELECT “library_book”.”title”, SUM(“library_book”.”count”) AS “total”
FROM “library_shelf” LEFT OUTER JOIN “library_book”
ON (“library_shelf”.”id” = “library_book”.”shelf_id”)
GROUP BY “library_book”.”title”, “library_book”.”title”
LIMIT 21′
PS:
The Sum I used in the query is a built-in function from Django, you can import it from django.db.models.
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