Custom Field Validations in Django Models

Custom field validations ensure that data entered into a model field meets specific rules before being saved to the database. They extend Django’s built-in validations, allowing developers to enforce additional rules.

• Can verify formats, length limits, or complex conditions (e.g., ensuring an email belongs to a certain domain).
• Run automatically when creating or updating model instances.
• Help maintain clean and consistent data without adding extra validation logic in forms or views.
• Builds on Django’s built-in field validations.

Syntax

field_name = models.Field(validators=[validator_function1, validator_function2])

• validators is a list of validator functions that will be executed when the field value is set.
• Each validator function accepts a single argument, value, which is the data entered for the field.

Example: A custom validator to only accept email addresses ending with @gmail.com. Consider a project named 'geeksforgeeks' having an app named 'geeks'.

1. Define the Model

In models.py, start with a simple model having a CharField for email:

from django.db import models
from django.db.models import Model

class GeeksModel(Model):
    geeks_mail = models.CharField(max_length = 200)

2. Create a Validator Function

Create a custom validator function that will check whether the email address ends with @gmail.com. If it doesn't, the function will raise a ValidationError.

from django.core.exceptions import ValidationError

def validate_geeks_mail(value):
    if not value.endswith("@gmail.com"):
        raise ValidationError("This field accepts only Gmail addresses.")

3. Attach the Validator to the Model Field

Attach this validator function to the geeks_mail field in our model using the validators parameter:

from django.db import models
from django.core.exceptions import ValidationError

def validate_geeks_mail(value):
    if not value.endswith("@gmail.com"):
        raise ValidationError("This field accepts only Gmail addresses.")

class GeeksModel(models.Model):
    geeks_mail = models.CharField(max_length=200, validators=[validate_geeks_mail])

Now, the geeks_mail field will only accept email addresses containing @gmail.com. Any other email address will trigger a ValidationError. Let us try to create an instance without gmail.com and check if our validation worked or not.

After every change in models.py, run these commands makemigrations and migrate commands to update the changes in database.

Visit: http://localhost:8000/admin/geeks/geeksmodel/add/ and enter "[email protected]":

The custom validator ensures that the geeks_mail field only accepts email addresses ending with @gmail.com. Any other email address triggers a ValidationError. This approach allows applying any kind of custom validation to a field’s value.