Ever built something with Django and thought, “I wish there was a field that could do exactly this”? I have. The built-in fields are fantastic, but sometimes your data has unique rules. That’s when you step beyond the manual clean() methods and create your own field. It’s not just about neat code—it’s about creating a self-contained, reusable component that knows how to validate, save, and represent your data from the database all the way to the form. Let’s build one together.

Think about a common need: storing sensitive text, like an API key, in the database. You wouldn’t want it sitting there in plain text. A custom field can handle the encryption and decryption automatically, every single time. How much cleaner is that than scattering encryption logic across your views and models?

Here’s the skeleton of a custom field. It starts by extending Django’s Field class.

from django.db import models

class BaseCustomField(models.Field):
    description = "My custom field"

    def __init__(self, *args, **kwargs):
        # Set up your field's unique options here
        super().__init__(*args, **kwargs)

    def deconstruct(self):
        # Crucial for migrations
        name, path, args, kwargs = super().deconstruct()
        return name, path, args, kwargs

The deconstruct method is your field’s recipe. Django uses it to remember how to recreate your field during migrations. If you add a custom parameter, like an encryption key, you must include it here.

The real magic happens in three key methods. They are the translators between your Python code and the database.

    def from_db_value(self, value, expression, connection):
        """Converts a value from the database into a Python object."""
        if value is None:
            return value
        return self.to_python(value)

    def to_python(self, value):
        """Converts a value from a form/serializer into the correct Python type."""
        if value is None or isinstance(value, list):
            return value
        return [item.strip() for item in value.split(',')]

    def get_prep_value(self, value):
        """Prepares the Python object for saving into the database."""
        if value is None:
            return None
        if isinstance(value, list):
            return ','.join(value)
        return value

See how to_python is the universal input converter? from_db_value calls it when reading from the database, but you could add special logic there, like decryption.

Let’s build that encrypted text field I mentioned. We’ll use a simple symmetric encryption from the cryptography library.

from cryptography.fernet import Fernet
from django.core.exceptions import ValidationError

class EncryptedTextField(models.TextField):
    def __init__(self, *args, **kwargs):
        # In a real project, get this from settings
        self.cipher = Fernet(b'your-32-url-safe-base64-key-here')
        super().__init__(*args, **kwargs)

    def get_prep_value(self, value):
        value = super().get_prep_value(value)
        if value:
            # Encrypt the string before saving
            return self.cipher.encrypt(value.encode()).decode()
        return value

    def from_db_value(self, value, expression, connection):
        if value:
            # Decrypt the string after reading
            return self.cipher.decrypt(value.encode()).decode()
        return value

Now, in your model, using api_key = EncryptedTextField() means the value is automatically encrypted when saved and decrypted when accessed. The rest of your code never has to worry about it. Isn’t that more reliable?

But what about forms? A custom field should know how to present itself. You can specify a custom widget.

    def formfield(self, **kwargs):
        from django import forms
        defaults = {'widget': forms.PasswordInput}
        defaults.update(kwargs)
        return super().formfield(**defaults)

This tells Django’s ModelForm to use a password input for our encrypted field, which is a sensible default.

Testing is non-negotiable. You must verify the full cycle: Python to database and back.

# test_fields.py
from django.test import TestCase
from .models import MyModel

class EncryptedFieldTest(TestCase):
    def test_encryption_cycle(self):
        secret = "SUPER_SECRET_123"
        obj = MyModel.objects.create(api_key=secret)
        obj.refresh_from_db()

        # The value we get back should be the original
        self.assertEqual(obj.api_key, secret)

        # Let's check the raw database value is NOT the plain text
        from django.db import connection
        with connection.cursor() as cursor:
            cursor.execute("SELECT api_key FROM myapp_mymodel WHERE id=%s", [obj.id])
            row = cursor.fetchone()
            self.assertNotEqual(row[0], secret)  # Should be encrypted gibberish

This test proves the field is working as designed: transparently securing data.

The biggest pitfall? Forgetting about migrations. If you change your field’s internal logic, like how you split a string, existing data in the database won’t change. You might need a data migration to update old records to the new format. Also, always consider database compatibility. Does your field’s db_type work with MySQL, PostgreSQL, and SQLite?

Building a custom field feels like giving Django a new, native superpower. It encapsulates complexity, promotes reuse, and makes your models incredibly declarative. You’re not just writing a field; you’re extending the framework itself. Once you’ve done it, you’ll start seeing opportunities everywhere.

Did this guide help you see the structure behind Django’s ORM? What kind of field would you build first? If you found this walk-through useful, please share it with another developer who might be stuck manually processing their data. Let me know in the comments what unique data problem you’re solving with a custom field