Refactor Event.to_come to speed it up

Use 4 simple SQL requests instead of a big one

Decreases complexity on SQL side by not JOINing the three inscription
sources in one request.

Previous function ran in 40 to 50 seconds
The new one runs in ~70 milliseconds

events/models.py

@@ -90,14 +90,35 @@ class Event(models.Model):
 
     @staticmethod
     def to_come(user):
-        events = [event for event in Event.objects.filter(
-            Q(model=False),
-            Q(end_inscriptions__gt=timezone.now()),
-            Q(inscriptions__user=user) | Q(private=False)
-        ).distinct().prefetch_related('formulas').order_by('start_time')
-         .annotate(reg_nb=(Count('inscriptions', distinct=True) + Count('extern_inscriptions', distinct=True) + Count('invitations', distinct=True)))
-         .extra(select={'ins': "(SELECT COUNT(*) FROM events_inscription WHERE events_inscription.event_id=events_event.id AND events_inscription.user_id=%d)" % user.id})]
-        return [(event.ins, event) for event in events]
+        events = list(
+            Event.objects.filter(
+                Q(model=False),
+                Q(end_inscriptions__gt=timezone.now()),
+                Q(inscriptions__user=user) | Q(private=False)
+            ).distinct().prefetch_related('formulas').order_by('start_time')
+            .extra(select={
+                # Pour savoir si l'utilisateur actuel est inscrit
+                'ins': "(SELECT COUNT(*) FROM events_inscription "
+                       "WHERE events_inscription.event_id=events_event.id "
+                       "AND events_inscription.user_id=%d)" % user.id
+            })
+        )
+        events_by_id = {e.pk: [e, 0] for e in events}
+        counts_ins = Event.objects.filter(pk__in=events_by_id.keys()).annotate(ins_cnt=Count('inscriptions'))
+        counts_ext = Event.objects.filter(pk__in=events_by_id.keys()).annotate(ext_cnt=Count('extern_inscriptions'))
+        counts_inv = Event.objects.filter(pk__in=events_by_id.keys()).annotate(inv_cnt=Count('invitations'))
+
+        for e in counts_ins:
+            events_by_id[e.pk][1] += e.ins_cnt
+        for e in counts_ext:
+            events_by_id[e.pk][1] += e.ext_cnt
+        for e in counts_inv:
+            events_by_id[e.pk][1] += e.inv_cnt
+
+        for e, cnt in events_by_id.values():
+            e.reg_nb = cnt
+
+        return [(e.ins, e) for e in events]
 
     def formulas_json(self):
         res = {f.name: "{} <br>(Cotisant: {}€ / Non cotisant: {}€)".format(f.name, f.price_contributor, f.price_non_contributor) for f in self.formulas.all()}