Faster filtering on submission API v2 endpoint

Instead of joining the user table and looking up the username, we can
simply look up the profile ID and use the index. A similar optimization
is done for problems. Fixes #1584.

judge/views/api/api_v2.py

@@ -1,7 +1,7 @@
 from operator import attrgetter
 
 from django.conf import settings
-from django.core.exceptions import PermissionDenied, ValidationError
+from django.core.exceptions import ObjectDoesNotExist, PermissionDenied, ValidationError
 from django.db.models import Count, F, OuterRef, Prefetch, Q, Subquery
 from django.http import Http404, JsonResponse
 from django.utils import timezone
@@ -18,6 +18,30 @@
 from judge.views.submission import group_test_cases
 
 
+class BaseSimpleFilter:
+    def __init__(self, lookup):
+        self.lookup = lookup
+
+    def get_object(self, key):
+        raise NotImplementedError()
+
+    def to_filter(self, key):
+        try:
+            return {self.lookup: self.get_object(key)}
+        except ObjectDoesNotExist:
+            return {self.lookup: None}
+
+
+class ProfileSimpleFilter(BaseSimpleFilter):
+    def get_object(self, key):
+        return Profile.objects.get(user__username=key)
+
+
+class ProblemSimpleFilter(BaseSimpleFilter):
+    def get_object(self, key):
+        return Problem.objects.get(code=key)
+
+
 class APILoginRequiredException(Exception):
     pass
 
@@ -108,10 +132,13 @@ def filter_queryset(self, queryset):
 
         for key, filter_name in self.basic_filters:
             if key in self.request.GET:
-                # May raise ValueError or ValidationError, but is caught in APIMixin
-                queryset = queryset.filter(**{
-                    filter_name: self.request.GET.get(key),
-                })
+                if isinstance(filter_name, BaseSimpleFilter):
+                    queryset = queryset.filter(**filter_name.to_filter(self.request.GET.get(key)))
+                else:
+                    # May raise ValueError or ValidationError, but is caught in APIMixin
+                    queryset = queryset.filter(**{
+                        filter_name: self.request.GET.get(key),
+                    })
                 self.used_basic_filters.add(key)
 
         for key, filter_name in self.list_filters:
@@ -516,8 +543,8 @@ def get_object_data(self, profile):
 class APISubmissionList(APIListView):
     model = Submission
     basic_filters = (
-        ('user', 'user__user__username'),
-        ('problem', 'problem__code'),
+        ('user', ProfileSimpleFilter('user')),
+        ('problem', ProblemSimpleFilter('problem')),
     )
     list_filters = (
         ('language', 'language__key'),