F and Q expressions in django

F() expression

Using F() objects can directly reference the values ​​of model fields and perform database operations instead of importing them into python 's memory and then performing operations, which can efficiently complete batch data operations.

for example

• Without using the F() expression, if we want to increase the price of all books by 50 yuan, we can write the code like this

  books = Book.objects.all()
  for book in books:
      book.price += 50
      book.save()
  

• Use F() expression combined with update() method to modify the price of books

  from django.db.models import F
  books = Book.objects.update(price=F('price')+50)
  

• Do not use F() expressions to query data information with the same value in the name field and emai field

  authors = Author.objects.all()
  for author in authors:
      if author.name == author.email:
          print(author)
  

• Use the F() expression to query data information with the same value in the name field and emai field

  from django.db.models import F
  authors = Author.objects.filter(name=F("email"))
  

Q() expression

The Q() expression can realize logical operations such as OR , & , NOT~ .

for example

• The query book id number is1Or title isDream of the Red ChamberBook information

  from django.db.models import Q
  books = Book.objects.filter(Q(id=1) | Q(name='红楼梦'))
  

• Queries book title isThree KingdomsAnd the price is98Book information

  from django.db.models import Q
  book = Book.objects.filter(Q(name='三国演义') & Q(price=98))
  

• Queries title is notThree KingdomsBook information

  from django.db.models import Q
  books = Book.objects.filter(~Q(name='三国演义'))