异步函数(通道使用者) Django中的数据库抓取模型

Question

摘要

你好，目前我正在开发一个应用程序，该应用程序通过django-通道在实时时将数据发送到前端。我已经能够为所有在打开页面时创建的模型这样做了，但是我还没有能够抓取以前的模型，因为页面还没有打开。

代码

下面是我第一次尝试让这个系统正常工作，这将在使用者连接方法被触发时调用，并且在连接被接受之后，我将运行这个for循环。

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

在运行这段代码时，我将得到django.core.exceptions.SynchronousOnlyOperation: You cannot call this from an async context - use a thread or sync_to_async.的错误消息

这是有意义的，因为使用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会正常工作，因为我已经在其他异步方法中使用了它

这导致了TypeError: 'coroutine' object is not iterable

最后的想法

我对django通道和异步通道都很陌生，如果我知道通道将以异步为主，那么在我开始这个项目之前，我就会研究这个系统。

我查看了其他堆栈溢出帖子，并发现可能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-通道的问题。

我还能做些什么？

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

这个代码sync_to_async(Application.objects.all)()创建了一个协同线，需要等待而不是直接使用。对于您的具体情况，最好先在For循环之外获取数据，然后继续。另外，您可能希望使用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})

另外，请注意，当序列化程序序列化应用实例时，请确保没有数据库查询。为此，请使用select_related和prefetch_related预取可能需要的嵌套对象。

UPDATE：是的，由于一些Django方法(如.all() )的惰性特性，数据库实际上不会在调用该方法时被击中，而是在它被使用时被击中。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})