In the Django QuerySet API, F() expressions are used to refer to model field values directly in the database.
Let’s say you have a Product class with a price field, and you want to increase the price of all products in 20%.
A possible solution would be:
products = Product.objects.all()
for product in products:
product.price *= 1.2
product.save()Instead you could use an F() expression to update it in a single query:
from django.db.models import F
Product.objects.update(price=F('price') * 1.2)You can also do it for a single object:
product = Product.objects.get(pk=5009)
product.price = F('price') * 1.2
product.save()But take care with this kind of assignment. The F() object persist after saving the model.
product.price # price = Decimal('10.00')
product.price = F('price') + 1
product.save() # price = Decimal('11.00')
product.name = 'What the F()'
product.save() # price = Decimal('12.00')So, basically after updating a field like that, product.price will hold an instance of
django.db.models.expressions.CombinedExpression, instead of the actual result. If you want to access the result
immediately:
product.price = F('price') + 1
product.save()
print(product.price) # <CombinedExpression: F(price) + Value(1)>
product.refresh_from_db()
print(product.price) # Decimal('13.00')You can also use it to annotate data:
from django.db.models import ExpressionWrapper, DecimalField
Product.objects.all().annotate(
value_in_stock=ExpressionWrapper(
F('price') * F('stock'), output_field=DecimalField()
)
)Since price is a DecimalField and stock is a IntegerField, we need to wrap the expression inside a
ExpressionWrapper object.
It can be used to filter data as well:
Product.objects.filter(stock__gte=F('ordered'))
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