django queryset高级用法

https://docs.djangoproject.com/en/dev/ref/models/expressions/

from django.db.models import Count, F, Value
from django.db.models.functions import Length, Upper

# Find companies that have more employees than chairs.
Company.objects.filter(num_employees__gt=F('num_chairs'))

# Find companies that have at least twice as many employees
# as chairs. Both the querysets below are equivalent.
Company.objects.filter(num_employees__gt=F('num_chairs') * 2)
Company.objects.filter(
    num_employees__gt=F('num_chairs') + F('num_chairs'))

# How many chairs are needed for each company to seat all employees?
>>> company = Company.objects.filter(
...    num_employees__gt=F('num_chairs')).annotate(
...    chairs_needed=F('num_employees') - F('num_chairs')).first()
>>> company.num_employees
120
>>> company.num_chairs
50
>>> company.chairs_needed
70

# Create a new company using expressions.
>>> company = Company.objects.create(name='Google', ticker=Upper(Value('goog')))
# Be sure to refresh it if you need to access the field.
>>> company.refresh_from_db()
>>> company.ticker
'GOOG'

# Annotate models with an aggregated value. Both forms
# below are equivalent.
Company.objects.annotate(num_products=Count('products'))
Company.objects.annotate(num_products=Count(F('products')))

# Aggregates can contain complex computations also
Company.objects.annotate(num_offerings=Count(F('products') + F('services')))

# Expressions can also be used in order_by(), either directly
Company.objects.order_by(Length('name').asc())
Company.objects.order_by(Length('name').desc())
# or using the double underscore lookup syntax.
from django.db.models import CharField
from django.db.models.functions import Length
CharField.register_lookup(Length)
Company.objects.order_by('name__length')

# Boolean expression can be used directly in filters.
from django.db.models import Exists
Company.objects.filter(
    Exists(Employee.objects.filter(company=OuterRef('pk'), salary__gt=10))
)