Django Template Iteration and Asynchronous Queries

Template Iteration and Asynchronous Behavior

The Django template's {% for %} loop is not asynchronous by default. When you pass a queryset to a template and iterate over it with a {% for %} loop, Django will execute the query synchronously at that point, which can block the event loop in an asynchronous context.

Making Your View Asynchronous

To ensure asynchronous iteration, you need to:

1. Make your view function/method asynchronous
2. Evaluate the queryset asynchronously before passing it to the template

Here's how you can modify your example:

PYTHONCopied
async def get_context_data(self):
    # get parent context
    queryset = Model.objects.filter(...)
    
    # Evaluate the queryset asynchronously and convert to a list
    objs = [obj async for obj in queryset]
    
    context['objs'] = objs
    return context

By using an async comprehension to evaluate the queryset before passing it to the template, you're ensuring that the database query happens asynchronously. The template then receives a regular list, not a queryset, so its synchronous iteration won't cause any blocking.

Alternative Approach

If you're using class-based views, you'll need to make sure the entire view is asynchronous:

PYTHONCopied
class MyAsyncView(View):
    async def get(self, request, *args, **kwargs):
        context = await self.get_context_data()
        # Render template with context
        
    async def get_context_data(self):
        context = {}
        queryset = Model.objects.filter(...)
        context['objs'] = [obj async for obj in queryset]
        return context

Important Notes

1. Remember that Django templates themselves don't have built-in async support - they always execute synchronously.

2. The key is to resolve any querysets asynchronously before they reach the template.

3. If you're using transactions, note that they don't yet work in async mode. As the documentation states, you would need to write transaction-dependent code as a synchronous function and call it using sync_to_async().

By following these patterns, you can ensure your database queries don't block the event loop while still using Django's template system normally.