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29 / 30Django Models and Database DesignHardPython - Django Framework

Django Model Validation: clean() Method vs Validators

from django.core.exceptions import ValidationError
from django.core.validators import MinValueValidator

class Product(models.Model):
    name = models.CharField(max_length=100)
    price = models.DecimalField(max_digits=10, decimal_places=2, validators=[MinValueValidator(0.01)])
    stock = models.IntegerField()
    
    def clean(self):
        if self.stock < 0:
            raise ValidationError('Stock cannot be negative')
        if self.price > 10000:
            raise ValidationError('Price cannot exceed 10000')

# Hari creates a product:
product = Product(name='Laptop', price=15000, stock=-5)
product.save()

  1. The save() will fail because the clean() method validates that price <= 10000 and stock >= 0.

  2. The save() will succeed because clean() is not automatically called by save(); you must explicitly call product.clean() before saving.

  3. The save() will fail because the MinValueValidator on price field prevents values over 10000.

  4. The save() will fail because Django automatically validates all fields defined in the model.

Show answer & explanation

Correct answer

The save() will succeed because clean() is not automatically called by save(); you must explicitly call product.clean() before saving.

Explanation

Field validators run during form validation and model validation (via clean()), but model.save() bypasses the clean() method. The clean() method must be explicitly called. The MinValueValidator will not prevent negative stock because it only applies to the price field.

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All 30 Django Models and Database Design questions

  1. 1.What is the primary purpose of Django Models?
  2. 2.Which field type should be used to store an email address in a Django Model?
  3. 3.What does the Meta class inside a Django Model do?
  4. 4.Which relationship type is used when one model can have many instances of another model?
  5. 5.What is the purpose of the `__str__` method in a Django Model?
  6. 6.Which field attribute makes a field mandatory in Django Models?
  7. 7.What does the `on_delete` parameter do in a ForeignKey relationship?
  8. 8.Which of the following is a valid Django field type for storing boolean values?
  9. 9.What is the default behavior when you don't specify `null=True` for a field in Django?
  10. 10.Which method is used to retrieve a single object from the database in Django?
  11. 11.Understanding Django Model Meta Options
  12. 12.Scenario:
  13. 13.Django QuerySet Filter vs Get Method
  14. 14.Coding:
  15. 15.Scenario:
  16. 16.Understanding Django Model Inheritance
  17. 17.Coding:
  18. 18.Scenario:
  19. 19.Django Model Manager and QuerySet Methods
  20. 20.Coding:
  21. 21.Django Model Inheritance Strategy Selection
  22. 22.Django QuerySet Optimization with select_related vs prefetch_related
  23. 23.Django Model Field Choices and Validation
  24. 24.Django Model Relationships:
  25. 25.Django Model Signals and Database Consistency
  26. 26.Django Model Meta Options:
  27. 27.Django Model Managers and Custom QuerySets
  28. 28.Django Model Field:
  29. 29.Django Model Validation:
  30. 30.Django Model Aggregation and Annotation