Encrypted Fields with Uniqueness Constraint in Django

The issue you're encountering is due to a fundamental conflict between encryption and database-level uniqueness constraints. When you encrypt a field, the database can't enforce uniqueness on the encrypted values in the way you'd expect.

Why This Happens

When you set unique=True on a field, Django enforces uniqueness at the database level. However, with encrypted fields:

1. The database sees only the encrypted values, not the original values
2. The same original value will have different encrypted representations each time
3. The database can't perform normal lookups on encrypted data

Possible Solutions

1. Use a Separate Unique Field

One approach is to maintain a separate field that stores a hash or other unique representation of the value:

PYTHONCopied
from django.db import models
from django_cryptography.fields import encrypt
from django.core.validators import MinValueValidator, MaxValueValidator
import hashlib

class MyModel(models.Model):
    # Encrypted field
    sin = encrypt(models.IntegerField(
        validators=[MinValueValidator(10000000), MaxValueValidator(99999999)],
        default="",
    ))
    # Hash field for uniqueness
    sin_hash = models.CharField(max_length=64, unique=True, editable=False)
    
    def save(self, *args, **kwargs):
        # Generate a hash of the SIN for uniqueness checking
        if self.sin:
            self.sin_hash = hashlib.sha256(str(self.sin).encode()).hexdigest()
        super().save(*args, **kwargs)

2. Application-Level Uniqueness Check

Implement uniqueness checking at the application level before saving:

PYTHONCopied
from django.db import models
from django_cryptography.fields import encrypt
from django.core.validators import MinValueValidator, MaxValueValidator
from django.core.exceptions import ValidationError

class MyModel(models.Model):
    sin = encrypt(models.IntegerField(
        validators=[MinValueValidator(10000000), MaxValueValidator(99999999)],
        default="",
    ))
    
    def clean(self):
        # Check for uniqueness at the application level
        if self.sin and MyModel.objects.filter(sin=self.sin).exclude(pk=self.pk).exists():
            raise ValidationError({'sin': 'This SIN already exists.'})
    
    def save(self, *args, **kwargs):
        self.full_clean()
        super().save(*args, **kwargs)

3. Use UUID as Primary Key

For some use cases, you might consider using a UUID as a primary key instead of relying on the encrypted field for uniqueness:

PYTHONCopied
import uuid
from django.db import models
from django_cryptography.fields import encrypt

class MyModel(models.Model):
    id = models.UUIDField(primary_key=True, default=uuid.uuid4, editable=False)
    sin = encrypt(models.IntegerField(
        validators=[MinValueValidator(10000000), MaxValueValidator(99999999)],
        default="",
    ))

Important Considerations

1. Performance: Application-level uniqueness checks can be slower than database-level constraints
2. Race conditions: Without database constraints, concurrent operations could potentially create duplicates
3. Security: Storing hashes of sensitive data may still expose some information, though much less than the original values

The best approach depends on your specific requirements for security, performance, and data integrity.