How to Use F() Expressions in Django

Table of Content

• How to Use F() Expressions in List of Objects
• How to Use F() Expressions in a Single Object
• How to Use F() Expressions to Annotate Data
• How to Use F() Expressions to Filter Data
• Drawback of Using F() Expressions in Django
• Wrap Off

The F() expressions in Django are very important additions that most people tend to ignore.

When working with the Django Query Set API, the F() expressions can be used to refer to model field values directly in the database.

Let's say you have a Book class with a quantity field, and you want to increase the quantity of the books by 50%.

One solution that most people often use is:

books = Book.objects.all()
for book in books:
    book.quantity *= 1.5
    book.save()

This solution, although it is possible and works, it can be replaced with an F() expression.

With an F() expression, the query can be updated in a single query.

How to Use F() Expressions in List of Objects

from django.db.models import F

Book.objects.update(quantity=F('quantity') * 1.5)

How to Use F() Expressions in a Single Object

You can also do it for a single object:

book = Book.objects.get(id=10)
book.quantity = F('quantity') * 1.5
book.save()

How to Use F() Expressions to Annotate Data

Here's how you can use F() expression to carry out data annotations.

from django.db.models import ExpressionWrapper

Book.objects.all().annotate(
    value_in_stock=ExpressionWrapper(
        F('quantity') * F('label'), output_field=IntegerField()
    )
)

Since quantity is an IntegerField and stock is a IntegerField, we need to wrap the expression inside a ExpressionWrapper object.

How to Use F() Expressions to Filter Data

Here's how it can be used to filter data as well:

Book.objects.filter(label__gte=F('ordered'))

Drawback of Using F() Expressions in Django

Before using this method in your project, you should know that the F() object persist after saving the model.

For instance:

book.quantity                   # quantity = int(10)
book.quantity = F('quantity') + 1
book.save()                  # quantity = int('11')
book.name = 'Testing F'
book.save()                  # quantity = int(12.00)

So, basically after updating a field like that, book.quantity will hold an instance of django.db.models.expressions.CombinedExpression, instead of the actual result. If you want to access the result immediately:

book.quantity = F('quantity') + 1
book.save()
print(book.quantity)            # <CombinedExpression: F(quantity) + Value(1)>
book.refresh_from_db()
print(book.quantity)            # int(13)

Wrap Off

There you have it! I hope you found this post useful somehow.

If you have any questions or need clarification, feel free to reach out to me.

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