Django can't handle m2m relations well sometimes.

[AvTiMp]

In my case, we have hundreds of pods who run gunicorn to handle requests and controlled by k8s hpa, we set that gunicorn should restart one worker's thread who has handled 10000 requests, so there could be thosands of restart per day. but something strange happens then, sometimes a gunicorn wroker thread report an error like this:

host    : dp-b6b59027f6-57f89f9547-4qpv5 
username:  
args    : () 
kwargs  : {} 
exc     : Traceback (most recent call last):
  File "/usr/local/lib/python2.7/dist-packages/rest_framework/views.py", line 471, in dispatch
    response = handler(request, *args, **kwargs)
  File "/opt/tiger/tce_platform/src/platform_site/service/views/service_v3.py", line 715, in get_by_psm
    result = serializer.data
  File "/usr/local/lib/python2.7/dist-packages/rest_framework/serializers.py", line 507, in data
    ret = super(Serializer, self).data
  File "/usr/local/lib/python2.7/dist-packages/rest_framework/serializers.py", line 239, in data
    self._data = self.to_representation(self.instance)
  File "/usr/local/lib/python2.7/dist-packages/rest_framework/serializers.py", line 476, in to_representation
    ret[field.field_name] = field.to_representation(attribute)
  File "/usr/local/lib/python2.7/dist-packages/rest_framework/serializers.py", line 615, in to_representation
    iterable = data.all() if isinstance(data, models.Manager) else data
  File "/usr/local/lib/python2.7/dist-packages/django/db/models/manager.py", line 223, in all
    return self.get_queryset()
  File "/usr/local/lib/python2.7/dist-packages/django/db/models/fields/related_descriptors.py", line 792, in get_queryset
    return qs._next_is_sticky().filter(**self.core_filters)
  File "/usr/local/lib/python2.7/dist-packages/django/db/models/query.py", line 790, in filter
    return self._filter_or_exclude(False, *args, **kwargs)
  File "/usr/local/lib/python2.7/dist-packages/django/db/models/query.py", line 808, in _filter_or_exclude
    clone.query.add_q(Q(*args, **kwargs))
  File "/usr/local/lib/python2.7/dist-packages/django/db/models/sql/query.py", line 1243, in add_q
    clause, _ = self._add_q(q_object, self.used_aliases)
  File "/usr/local/lib/python2.7/dist-packages/django/db/models/sql/query.py", line 1269, in _add_q
    allow_joins=allow_joins, split_subq=split_subq,
  File "/usr/local/lib/python2.7/dist-packages/django/db/models/sql/query.py", line 1149, in build_filter
    lookups, parts, reffed_expression = self.solve_lookup_type(arg)
  File "/usr/local/lib/python2.7/dist-packages/django/db/models/sql/query.py", line 1035, in solve_lookup_type
    _, field, _, lookup_parts = self.names_to_path(lookup_splitted, self.get_meta())
  File "/usr/local/lib/python2.7/dist-packages/django/db/models/sql/query.py", line 1330, in names_to_path
    "Choices are: %s" % (name, ", ".join(available)))
FieldError: Cannot resolve keyword u'service' into field. Choices are: id, is_primary, port, type

then all same kind of requests proxying to this failed with the same error, but other threads work just as excepted.

To find the reason, I echo the field info of all models at our handler func when error occurs, like this:

from django.core.exceptions import FieldError
try:
    ##handler logic here
    ##just try to get one instance of model service who has many ManyToManyFields(with model B, C, D ...). they all belongs to app service
except FieldError as e:
    from django.apps import apps
    for v in apps.get_app_config('service').get_models(include_auto_created=True, include_deferred=True):
        print('-----------------', v.__name__) 
        # <class 'django.utils.datastructures.ImmutableList'>
        print('       ----------', v._meta.get_fields())
    raise e

then I found that model B has no field <ManyToManyRel: service.service>, so we have the previous error Cannot resolve keyword u'service' into field.
but in other healthy threads they all have this field. I think the func _populate_directed_relation_graph generate all relate fields then store them in __dict__ which leading to the errors of all coming requests. So why does this happen?

[Mariusz Felisiak]

Thanks for this report, however Django 1.9 is not supported anymore, we can consider this as a bug if you will be able to reproduce this in Django 2.2+. Moreover you use case looks niche and can be hard to reproduce, so a sample project is also required.