在异步函数中从数据库中抓取模型（Channel Consumer）Django

Question

总结

您好，目前我正在开发一个应用程序，它通过使用 django-channels 将数据实时发送到前端。我已经能够对页面打开时创建的所有模型执行此操作，但是我还无法在页面尚未打开时获取以前的模型。

代码

下面是我第一次尝试让这个系统正常工作，当消费者连接方法被触发时会调用它，在连接被接受后我会运行这个 for 循环

for app in Application.objects.all():
    app_json = ApplicationSerializer(app).data
    await self.send_json({'action': 'create', 'data': app_json})

当运行使用这段代码时，我会得到错误信息 django.core.exceptions.SynchronousOnlyOperation: You cannot call this from an async context - use a thread or sync_to_async.

这是有道理的，因为消费者连接方法是异步的，所以按照消息的建议我继续使用 sync_to_async

for app in sync_to_async(Application.objects.all)():
    app_json = ApplicationSerializer(app).data
    await self.send_json({'action': 'create', 'data': app_json})

我决定在 Application 对象周围使用 sync_to_async，因为错误消息突出显示了 for 循环行本身，而且我知道 applicationSerializer 可以正常工作，因为我已经在其他异步方法中使用它了

这导致 TypeError: 'coroutine' object is not iterable

最后的想法

我对 django-channels 和异步都是新手，如果我知道通道将在很大程度上基于异步，我会在开始这个项目之前研究这个系统。

我查看了其他堆栈溢出帖子，发现潜在映射数据可能是一种解决方案，但我不能 100% 确定这是异步问题还是 django-channels

还有哪些我可以尝试的东西？

更新

根据 Kens 的建议，我继续尝试了以下代码：

   apps = await database_sync_to_async(Application.objects.all)()
   for app in apps:
       app_json = ApplicationSerializer(app).data
       print(app_json)

这返回了堆栈跟踪

Exception inside application: You cannot call this from an async context - use a thread or sync_to_async.
Traceback (most recent call last):
  File "C:\Users\Devin\.virtualenvs\HomeAutomation-l9uhKhE0\lib\site-packages\channels\staticfiles.py", line 44, in __call__
    return await self.application(scope, receive, send)
  File "C:\Users\Devin\.virtualenvs\HomeAutomation-l9uhKhE0\lib\site-packages\channels\routing.py", line 71, in __call__
    return await application(scope, receive, send)
  File "C:\Users\Devin\.virtualenvs\HomeAutomation-l9uhKhE0\lib\site-packages\channels\sessions.py", line 47, in __call__
    return await self.inner(dict(scope, cookies=cookies), receive, send)
  File "C:\Users\Devin\.virtualenvs\HomeAutomation-l9uhKhE0\lib\site-packages\channels\sessions.py", line 254, in __call__
    return await self.inner(wrapper.scope, receive, wrapper.send)
  File "C:\Users\Devin\.virtualenvs\HomeAutomation-l9uhKhE0\lib\site-packages\channels\auth.py", line 181, in __call__
    return await super().__call__(scope, receive, send)
  File "C:\Users\Devin\.virtualenvs\HomeAutomation-l9uhKhE0\lib\site-packages\channels\middleware.py", line 26, in __call__
    return await self.inner(scope, receive, send)
  File "C:\Users\Devin\.virtualenvs\HomeAutomation-l9uhKhE0\lib\site-packages\channels\routing.py", line 150, in __call__
    return await application(
  File "C:\Users\Devin\.virtualenvs\HomeAutomation-l9uhKhE0\lib\site-packages\channels\consumer.py", line 94, in app
    return await consumer(scope, receive, send)
  File "C:\Users\Devin\.virtualenvs\HomeAutomation-l9uhKhE0\lib\site-packages\channels\consumer.py", line 58, in __call__
    await await_many_dispatch(
  File "C:\Users\Devin\.virtualenvs\HomeAutomation-l9uhKhE0\lib\site-packages\channels\utils.py", line 51, in await_many_dispatch
    await dispatch(result)
  File "C:\Users\Devin\.virtualenvs\HomeAutomation-l9uhKhE0\lib\site-packages\channels\consumer.py", line 73, in dispatch
    await handler(message)
  File "C:\Users\Devin\.virtualenvs\HomeAutomation-l9uhKhE0\lib\site-packages\channels\generic\websocket.py", line 196, in websocket_receive
    await self.receive(text_data=message["text"])
  File "C:\Users\Devin\.virtualenvs\HomeAutomation-l9uhKhE0\lib\site-packages\channels\generic\websocket.py", line 259, in receive
    await self.receive_json(await self.decode_json(text_data), **kwargs)
  File "D:\HomeAutomation\HomeAutomation\consumers.py", line 28, in receive_json
    for app in apps:
  File "C:\Users\Devin\.virtualenvs\HomeAutomation-l9uhKhE0\lib\site-packages\django\db\models\query.py", line 287, in __iter__
    self._fetch_all()
  File "C:\Users\Devin\.virtualenvs\HomeAutomation-l9uhKhE0\lib\site-packages\django\db\models\query.py", line 1308, in _fetch_all
    self._result_cache = list(self._iterable_class(self))
  File "C:\Users\Devin\.virtualenvs\HomeAutomation-l9uhKhE0\lib\site-packages\django\db\models\query.py", line 53, in __iter__
    results = compiler.execute_sql(chunked_fetch=self.chunked_fetch, chunk_size=self.chunk_size)
  File "C:\Users\Devin\.virtualenvs\HomeAutomation-l9uhKhE0\lib\site-packages\django\db\models\sql\compiler.py", line 1154, in execute_sql
    cursor = self.connection.cursor()
  File "C:\Users\Devin\.virtualenvs\HomeAutomation-l9uhKhE0\lib\site-packages\django\utils\asyncio.py", line 24, in inner
    raise SynchronousOnlyOperation(message)
django.core.exceptions.SynchronousOnlyOperation: You cannot call this from an async context - use a thread or sync_to_async.

Answer 1

这段代码sync_to_async(Application.objects.all)()创建了一个协程，需要等待，不能直接使用。对于您的具体情况，最好先在 for 循环之外获取数据，然后再继续。另外，您可能想改用 database_sync_to_async。

from channels.db import database_sync_to_async

apps = await database_sync_to_async(Application.objects.all)()
for app in apps:
    app_json = ApplicationSerializer(app).data
    await self.send_json({'action': 'create', 'data': app_json})

附带说明一下，确保在序列化程序序列化应用程序实例时不会有数据库查询。为此，请使用 select_related 和 prefetch_related 预取可能需要的任何嵌套对象

UPDATE：是的，由于某些 Django ORM 方法的惰性，例如 .all()，在调用方法，但在使用它的时候。强制它立即执行的一种常见方法是将查询集转换为列表。试试这个：

from channels.db import database_sync_to_async

apps = await database_sync_to_async(list)(Application.objects.all())
for app in apps:
    app_json = ApplicationSerializer(app).data
    await self.send_json({'action': 'create', 'data': app_json})

在异步函数中从数据库中抓取模型（Channel Consumer）Django

Grabbing models from database in asynchronous function (Channel Consumer) Django

python

django

asynchronous

django-channels

总结

代码

最后的想法

更新