Django query ForeignKey Count() zero

问题

I have 3 tables:

Truck with the fields: id, name....

Menu with the fields: id, itemname, id_foodtype, id_truck...

Foodtype with the fields: id, type...

I want to get a summary like:

id  name              total
10  Alcoholic drink   0
5   Appetizer         11

My problem is to return the results with 0 elements.

I tried an SQL query like this:

SELECT
    ft.id, ft.name, COUNT(me.id) total
FROM
    foodtype ft LEFT JOIN menu me
        ON ft.id = me.id_foodtype
    LEFT JOIN truck tr
        ON tr.id = me.id_truck AND tr.id = 3
GROUP BY ft.id, ft.name
ORDER BY ft.name

or a query in Django

Menu.objects.filter(id_truck=3).values("id_foodtype").annotate(cnt=Count("id_foodtype"))

But, neither is displaying the results with Zero elements.

At the moment to convert this query to Python code, any of my queries return the exact result that I expected. How can I return results with the Left Join including the foodtypes with zero elements in the menu?

回答1:

The direction of LEFT JOIN depends on the object, where you start the query. If it start on Menu you will never see a FoodType unused by selected Menu items. Then is important to filter (by Truck in your case) such way that also null value Menu.id is allowed in order to can get Count == 0.

from django.db.models import Q
qs = (
    FoodType.objects
    .filter(Q(menu_set__id_truck=3) | Q(menu_set__id__isnull=True))
    .values()   # not necessary, but useful if you want a dict, not a Model object
    .annotate(cnt=models.Count("menu_set__id"))
)

Verify:

>>> print(str(qs.query))
SELECT foodtype.id, foodtype..., COUNT(menu.id) AS cnt
FROM foodtype
LEFT OUTER JOIN menu ON (foodtype.id = menu.id_foodtype)
WHERE _menu.id_truck = 3 OR menu.id IS NULL)
GROUP BY foodtype.id

It works with the current newest and oldest Django 2.0b1 and 1.8.

The query is the same with or without the line .values(). The results are dictionaries or FoodType objects with a cnt attribute.

Footnotes:
The name menu_set should be replaced by the real related_name of foreign key id_foodtype if you have defined the related_name.

class Menu(models.Model):
    id_foodtype = models.ForeignKey('FoodType', on_delete=models.DO_NOTHING,
                                    db_column='id_foodtype', related_name='menu_set'))
    ...

If you start a new project I recommend to rename the foreign key to a name without "id" and the db_column field is with "id". Then menu_item.foodtype is a Food object and menu_item.id_foodtype its id.

来源：https://stackoverflow.com/questions/46838375/django-query-foreignkey-count-zero

标签

django

django-queryset