FOR UPDATE cannot be applied to the nullable side of an outer join

[egegunes]

Since Postgresql doesn't support FOR UPDATE in nullable side of outer join (see here), caching access token after introspection throws this error on master branch:

Postgresql:

ERROR:  FOR UPDATE cannot be applied to the nullable side of an outer join                                                                                                                          
STATEMENT:  SELECT "oauth2_provider_accesstoken"."id", "oauth2_provider_accesstoken"."user_id", "oauth2_provider_accesstoken"."source_refresh_token_id", "oauth2_provider_accesstoken"."token",     "oauth2_provider_accesstoken"."application_id", "oauth2_provider_accesstoken"."expires", "oauth2_provider_accesstoken"."scope", "oauth2_provider_accessto ken"."created",                           "oauth2_provider_accesstoken"."updated", "auth_user"."id", "auth_user"."password", "auth_user"."last_login", "auth_user"."is_superuser", "auth_user"."username", "auth_user"."first_ name",         "auth_user"."last_name", "auth_user"."email", "auth_user"."is_staff", "auth_user"."is_active", "auth_user"."date_joined", "oauth2_provider_application"."id", "oauth2_provider_application"."       client_id", "oauth2_provider_application"."user_id", "oauth2_provider_application"."redirect_uris", "oauth2_provider_application"."client_type", "oauth2_provider_application".                     "authorization_grant_ type", "oauth2_provider_application"."client_secret", "oauth2_provider_application"."name", "oauth2_provider_application"."skip_authorization", "oauth2_provider_application"."created", "oauth2_pro vider_application"."updated" FROM "oauth2_provider_accesstoken" LEFT OUTER JOIN "auth_user" ON ("oauth2_provider_accesstoken"."user_id" = "auth_user"."id") LEFT OUTER JOIN  "oauth2_provider_application" ON ("oauth2_provider_accesstoken"."application_id" = "oauth2_provider_application"."id") WHERE "oauth2_provider_accesstoken"."token" = 'TOKEN' FOR UPDATE

Django:

Traceback (most recent call last):                                                                                                                                                                  
  File "/usr/local/lib/python3.7/site-packages/django/core/handlers/exception.py", line 34, in inner                                                                                                
    response = get_response(request)                                                                                                                                                                
  File "/usr/local/lib/python3.7/site-packages/django/core/handlers/base.py", line 126, in _get_response                                                                                            
    response = self.process_exception_by_middleware(e, request)                                                                                                                                     
  File "/usr/local/lib/python3.7/site-packages/django/core/handlers/base.py", line 124, in _get_response                                                                                            
    response = wrapped_callback(request, *callback_args, **callback_kwargs)                                                                                                                         
  File "/usr/local/lib/python3.7/site-packages/django/views/decorators/csrf.py", line 54, in wrapped_view                                                                                           
    return view_func(*args, **kwargs)                                                                                                                                                               
  File "/usr/local/lib/python3.7/site-packages/django/views/generic/base.py", line 68, in view                                                                                                      
    return self.dispatch(request, *args, **kwargs)                                                                                                                                                  
  File "/usr/local/lib/python3.7/site-packages/rest_framework/views.py", line 495, in dispatch                                                                                                      
    response = self.handle_exception(exc)                                                                                                                                                           
  File "/usr/local/lib/python3.7/site-packages/rest_framework/views.py", line 455, in handle_exception                                                                                              
    self.raise_uncaught_exception(exc)                                                                                                                                                              
  File "/usr/local/lib/python3.7/site-packages/rest_framework/views.py", line 483, in dispatch                                                                                                      
    self.initial(request, *args, **kwargs)                                                                                                                                                          
  File "/usr/local/lib/python3.7/site-packages/rest_framework/views.py", line 400, in initial                                                                                                       
    self.perform_authentication(request)                                                                                                                                                            
  File "/usr/local/lib/python3.7/site-packages/rest_framework/views.py", line 326, in perform_authentication                                                                                        
    request.user                                                                                                                                                                                    
  File "/usr/local/lib/python3.7/site-packages/rest_framework/request.py", line 223, in user                                                                                                        
    self._authenticate()                                                                                                                                                                            
  File "/usr/local/lib/python3.7/site-packages/rest_framework/request.py", line 376, in _authenticate                                                                                               
    user_auth_tuple = authenticator.authenticate(self)                                                                                                                                              
  File "/usr/local/lib/python3.7/site-packages/oauth2_provider/contrib/rest_framework/authentication.py", line 29, in authenticate                                                                  
    valid, r = oauthlib_core.verify_request(request, scopes=[])                                                                                                                                     
  File "/usr/local/lib/python3.7/site-packages/oauth2_provider/oauth2_backends.py", line 166, in verify_request                                                                                     
    valid, r = self.server.verify_request(uri, http_method, body, headers, scopes=scopes)                                                                                                           
  File "/usr/local/lib/python3.7/site-packages/oauthlib/oauth2/rfc6749/endpoints/base.py", line 87, in wrapper                                                                                      
    return f(endpoint, uri, *args, **kwargs)                                                                                                                                                        
  File "/usr/local/lib/python3.7/site-packages/oauthlib/oauth2/rfc6749/endpoints/resource.py", line 75, in verify_request                                                                           
    return token_type_handler.validate_request(request), request                                                                                                                                    
  File "/usr/local/lib/python3.7/site-packages/oauthlib/oauth2/rfc6749/tokens.py", line 351, in validate_request                                                                                    
    token, request.scopes, request)                                                                                                                                                                 
  File "/usr/local/lib/python3.7/site-packages/oauth2_provider/oauth2_validators.py", line 370, in validate_bearer_token                                                                            
    introspection_credentials                                                                                                                                                                       
  File "/usr/local/lib/python3.7/site-packages/oauth2_provider/oauth2_validators.py", line 342, in _get_token_from_authentication_server                                                            
    "expires": expires,                                                                                                                                                                             
  File "/usr/local/lib/python3.7/site-packages/django/db/models/query.py", line 502, in update_or_create                                                                                            
    obj = self.select_for_update().get(**lookup)                                                                                                                                                    
  File "/usr/local/lib/python3.7/site-packages/django/db/models/query.py", line 393, in get                                                                                                         
    num = len(clone)                                                                                                                                                                                
  File "/usr/local/lib/python3.7/site-packages/django/db/models/query.py", line 250, in __len__                                                                                                     
    self._fetch_all()                                                                                                                                                                               
  File "/usr/local/lib/python3.7/site-packages/django/db/models/query.py", line 1186, in _fetch_all                                                                                                 
    self._result_cache = list(self._iterable_class(self))                                                                                                                                           
  File "/usr/local/lib/python3.7/site-packages/django/db/models/query.py", line 54, in __iter__                                                                                                     
    results = compiler.execute_sql(chunked_fetch=self.chunked_fetch, chunk_size=self.chunk_size)                                                                                                    
  File "/usr/local/lib/python3.7/site-packages/django/db/models/sql/compiler.py", line 1065, in execute_sql                                                                                         
    cursor.execute(sql, params)                                                                                                                                                                     
  File "/usr/local/lib/python3.7/site-packages/django/db/backends/utils.py", line 100, in execute                                                                                                   
    return super().execute(sql, params)                                                                                                                                                             
  File "/usr/local/lib/python3.7/site-packages/django/db/backends/utils.py", line 68, in execute                                                                                                    
    return self._execute_with_wrappers(sql, params, many=False, executor=self._execute)                                                                                                             
  File "/usr/local/lib/python3.7/site-packages/django/db/backends/utils.py", line 77, in _execute_with_wrappers                                                                                     
    return executor(sql, params, many, context)                                                                                                                                                     
  File "/usr/local/lib/python3.7/site-packages/django/db/backends/utils.py", line 85, in _execute                                                                                                   
    return self.cursor.execute(sql, params)                                                                                                                                                         
  File "/usr/local/lib/python3.7/site-packages/django/db/utils.py", line 89, in __exit__                                                                                                            
    raise dj_exc_value.with_traceback(traceback) from exc_value                                                                                                                                     
  File "/usr/local/lib/python3.7/site-packages/django/db/backends/utils.py", line 85, in _execute                                                                                                   
    return self.cursor.execute(sql, params)                                                                                                                                                         
  django.db.utils.NotSupportedError: FOR UPDATE cannot be applied to the nullable side of an outer join

Removing select_related fixes the issue.