Django Admin Async Actions and Message Display Issue

The issue you're experiencing with messages not appearing when using async admin actions is related to how Django's message framework works with asynchronous code.

The Problem

When you use sync_to_async to call modeladmin.message_user() in an asynchronous context, the message is likely being stored correctly, but the issue is with how Django's message framework handles the request/response cycle in async contexts.

Solution Approaches

1. Use a Hybrid Approach

The most reliable approach is to keep your action handler synchronous but use async functions for the time-consuming operations:

PYTHONCopied
from asgiref.sync import async_to_sync
from django.contrib import admin, messages
from django.utils.translation import ngettext

@admin.action(description="Re-Sync selected customers from API")
def sync_customers(modeladmin, request, queryset):
    """Sync the selected customers from API"""
    # Define an async function for the bulk of the work
    async def perform_sync():
        for customer in queryset:
            customer_data = await api_client.get_customers(customer.id)
            await save_customer(customer_data)
    
    # Run the async function synchronously
    async_to_sync(perform_sync)()
    
    # Send the message in the synchronous context
    modeladmin.message_user(
        request,
        ngettext(
            '%d customer was successfully synced.',
            '%d customers were successfully synced.',
            len(queryset),
        ) % len(queryset), 
        messages.SUCCESS
    )

2. Use Django's Messages Framework Directly

If you're determined to use a fully async approach, you might try using the messages framework directly instead of through modeladmin.message_user():

PYTHONCopied
from django.contrib import admin, messages
from django.utils.translation import ngettext
from asgiref.sync import sync_to_async

@admin.action(description="Re-Sync selected customers from API")
async def sync_customers(modeladmin, request, queryset):
    """Sync the selected customers from API"""
    for customer in queryset:
        customer = await api_client.get_customers(customer.id)
        await save_customer(customer)
    
    # Use the messages framework directly
    await sync_to_async(messages.add_message)(
        request,
        messages.SUCCESS,
        ngettext(
            '%d customer was successfully synced.',
            '%d customers were successfully synced.',
            len(queryset),
        ) % len(queryset)
    )

Why This Happens

The Django admin interface and message framework were primarily designed for synchronous operations. When you use async functions, the message might be added to the request, but the admin view might not properly process it in the response cycle.

The hybrid approach (option 1) is generally more reliable because it ensures that the message is added in the synchronous context where Django's admin interface expects it, while still allowing you to benefit from async operations for the time-consuming parts of your code.