django expressionwrapper example

The Django ExpressionWrapper is a class that allows you to create complex expressions using database fields and operators. Here is an example that demonstrates the usage of ExpressionWrapper:

from django.db.models import F, ExpressionWrapper, DecimalField

# Assuming we have a model called Product with price and discount fields
products = Product.objects.annotate(
    discounted_price=ExpressionWrapper(
        F('price') - F('discount'),
        output_field=DecimalField(max_digits=10, decimal_places=2)
    )
)

# Accessing the discounted_price field of a product
product = products.first()
discounted_price = product.discounted_price

In this example, we import the necessary classes from Django, including F, which allows us to reference database fields within expressions. We also import ExpressionWrapper and DecimalField.

Next, we use the annotate() method on the Product model to add a new field called discounted_price. Inside the ExpressionWrapper, we subtract the value of the discount field from the value of the price field using the F() expression. We also specify the output_field as DecimalField with a maximum of 10 digits and 2 decimal places. This ensures that the result of the expression is of the correct data type.

Finally, we can access the discounted_price field of a product like any other field. In this example, we retrieve the first product from the products queryset and assign its discounted_price to the discounted_price variable.

This example showcases how to use the Django ExpressionWrapper to create complex expressions involving database fields.