django docs case when

Explanation of Django Docs "case when"

The Case and When expressions in Django allow you to perform conditional logic within a query. Here's an explanation of each step:

  1. Import the necessary modules:

  2. Import the Case and When classes from django.db.models.

  3. Construct the query:

  4. Use the annotate method on the queryset to add a new field that represents the conditional logic.

  5. Use the Case class to define the conditional logic, and the When class to specify the conditions and their corresponding values.

  6. Define the conditions:

  7. Use the When class to define the conditions for the Case expression.

  8. Each When instance takes a condition and a value to return if the condition is met.

  9. Apply the conditions:

  10. Use the Case class to apply the defined conditions and their corresponding values.

  11. Pass the list of When instances to the Case class, and specify the default value if none of the conditions are met.

  12. Use the annotated field:

  13. The annotated field can be used in the query for filtering, ordering, or displaying the results.

  14. Example:

  15. An example query using Case and When might look like: ```python from django.db.models import Case, When, IntegerField from myapp.models import MyModel

    MyModel.objects.annotate( status_text=Case( When(status='A', then='Active'), When(status='I', then='Inactive'), default='Unknown', output_field=CharField(), ) ) `` - In this example, a new fieldstatus_textis added toMyModel, which represents the status in text form based on the conditions defined usingCaseandWhen`.

This allows for conditional logic to be applied directly in the database query, providing flexibility and efficiency in handling different cases within the data.