Using aexists() in Async Views with Unique Field Validation

You're facing a common challenge when working with async views in Django that need to validate unique fields. Django's built-in uniqueness validators use the synchronous .exists() method, which causes errors in async contexts.

Solution Options

1. Create a Custom Async Validator

You can create a custom validator that uses aexists() for async contexts:

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from asgiref.sync import sync_to_async
from django.core.exceptions import ValidationError
from django.utils.translation import gettext_lazy as _

class AsyncUniqueValidator:
    def __init__(self, queryset, message=None):
        self.queryset = queryset
        self.message = message or _('This value must be unique.')

    async def __call__(self, value):
        # Filter the queryset to find potential duplicates
        qs = self.queryset.filter(field_name=value)
        
        # Use aexists() in async context
        if await qs.aexists():
            raise ValidationError(self.message)

2. Use sync_to_async as a Wrapper

For existing models with uniqueness constraints, you can wrap the validation in sync_to_async:

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from asgiref.sync import sync_to_async
from django.db import models

class MyModel(models.Model):
    unique_field = models.CharField(max_length=100, unique=True)
    
    # Add a method that can be called from async views
    @classmethod
    async def async_validate_unique_field(cls, value, exclude_id=None):
        # Wrap the exists check in sync_to_async
        qs = cls.objects.filter(unique_field=value)
        if exclude_id:
            qs = qs.exclude(id=exclude_id)
        
        exists = await sync_to_async(qs.exists, thread_sensitive=True)()
        return not exists

3. Adapt Your View Logic

In your async view, you can handle the validation manually:

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async def my_async_view(request):
    # Get the value to check
    value = request.POST.get('unique_field')
    
    # Check uniqueness using aexists()
    is_unique = not await YourModel.objects.filter(unique_field=value).aexists()
    
    if is_unique:
        # Safe to create/update
        # ...
    else:
        # Handle validation error
        # ...

Supporting Both Sync and Async Views

Since you need to support both sync and REST API views, you have a few options:

1. Use database constraints: Let the database handle uniqueness and catch the IntegrityError

2. Create dual validation methods:

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class MyModel(models.Model):
    unique_field = models.CharField(max_length=100, unique=True)
    
    @classmethod
    def validate_unique_field_sync(cls, value, exclude_id=None):
        qs = cls.objects.filter(unique_field=value)
        if exclude_id:
            qs = qs.exclude(id=exclude_id)
        return not qs.exists()
    
    @classmethod
    async def validate_unique_field_async(cls, value, exclude_id=None):
        qs = cls.objects.filter(unique_field=value)
        if exclude_id:
            qs = qs.exclude(id=exclude_id)
        return not await qs.aexists()

3. Create a utility function that detects context:

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import inspect
from asgiref.sync import sync_to_async

def validate_unique(queryset, is_async=None):
    """Validate uniqueness in both sync and async contexts"""
    if is_async is None:
        # Auto-detect if we're in an async context
        is_async = inspect.iscoroutinefunction(inspect.currentframe().f_back.f_code)
    
    if is_async:
        return queryset.aexists()
    else:
        return queryset.exists()

Remember that Django's built-in model validation will still use the synchronous methods, so you'll need to handle the validation explicitly in your async views or forms.