Goal: Give Django users the ability to explicitly disallow DB queries using a with context manager.
This repo shows a proof of concept implementation using the same approach that is used within SimpleTestCase to prevent DB access
All the interesting code is in core/views.py
Based on the views defined in core/views.py:
# /
def index(request: HttpRequest) -> HttpResponse:
book = Book.objects.select_related("author").last()
with disallow_db_connections():
# This will work, because all data is already loaded
# and we **don't** need to make another DB query to
# get the author name
serialized_author = f"Author: {book.author.name}"
return HttpResponse(serialized_author)
# /error/
def error_example(request: HttpRequest) -> HttpResponse:
book = Book.objects.last()
with disallow_db_connections():
# This will raise an exception because another DB query
# would have to be made to get the author name
serialized_author = f"Author: {book.author.name}"
return HttpResponse(serialized_author)curl localhost:8000 works as normal with no error
curl localhost:8000/error/ results in the following error:
Internal Server Error: /error/
Traceback (most recent call last):
File "/Users/wilhelmklopp/workspace/kolofordjango/disallow-db-connection-demo/.venv/lib/python3.8/site-packages/django/db/models/fields/related_descriptors.py", line 173, in __get__
rel_obj = self.field.get_cached_value(instance)
File "/Users/wilhelmklopp/workspace/kolofordjango/disallow-db-connection-demo/.venv/lib/python3.8/site-packages/django/db/models/fields/mixins.py", line 15, in get_cached_value
return instance._state.fields_cache[cache_name]
KeyError: 'author'
During handling of the above exception, another exception occurred:
Traceback (most recent call last):
File "/Users/wilhelmklopp/workspace/kolofordjango/disallow-db-connection-demo/.venv/lib/python3.8/site-packages/django/core/handlers/exception.py", line 47, in inner
response = get_response(request)
File "/Users/wilhelmklopp/workspace/kolofordjango/disallow-db-connection-demo/.venv/lib/python3.8/site-packages/django/core/handlers/base.py", line 181, in _get_response
response = wrapped_callback(request, *callback_args, **callback_kwargs)
File "/Users/wilhelmklopp/workspace/kolofordjango/disallow-db-connection-demo/core/views.py", line 59, in error_example
serialized_author = f"Author: {book.author.name}"
File "/Users/wilhelmklopp/workspace/kolofordjango/disallow-db-connection-demo/.venv/lib/python3.8/site-packages/django/db/models/fields/related_descriptors.py", line 187, in __get__
rel_obj = self.get_object(instance)
File "/Users/wilhelmklopp/workspace/kolofordjango/disallow-db-connection-demo/.venv/lib/python3.8/site-packages/django/db/models/fields/related_descriptors.py", line 154, in get_object
return qs.get(self.field.get_reverse_related_filter(instance))
File "/Users/wilhelmklopp/workspace/kolofordjango/disallow-db-connection-demo/.venv/lib/python3.8/site-packages/django/db/models/query.py", line 431, in get
num = len(clone)
File "/Users/wilhelmklopp/workspace/kolofordjango/disallow-db-connection-demo/.venv/lib/python3.8/site-packages/django/db/models/query.py", line 262, in __len__
self._fetch_all()
File "/Users/wilhelmklopp/workspace/kolofordjango/disallow-db-connection-demo/.venv/lib/python3.8/site-packages/django/db/models/query.py", line 1324, in _fetch_all
self._result_cache = list(self._iterable_class(self))
File "/Users/wilhelmklopp/workspace/kolofordjango/disallow-db-connection-demo/.venv/lib/python3.8/site-packages/django/db/models/query.py", line 51, in __iter__
results = compiler.execute_sql(chunked_fetch=self.chunked_fetch, chunk_size=self.chunk_size)
File "/Users/wilhelmklopp/workspace/kolofordjango/disallow-db-connection-demo/.venv/lib/python3.8/site-packages/django/db/models/sql/compiler.py", line 1173, in execute_sql
cursor = self.connection.cursor()
File "/Users/wilhelmklopp/workspace/kolofordjango/disallow-db-connection-demo/core/views.py", line 17, in __call__
raise AssertionError(self.message)
AssertionError: Database queries to 'default' are not allowed in SimpleTestCase subclasses. Either subclass TestCase or TransactionTestCase to ensure proper test isolation or add 'default' to django.test.testcases.SimpleTestCase.databases to silence this failure.
[06/Jun/2021 09:31:11] "GET /error/ HTTP/1.1" 500 105616