Alternate approach

dashboard/models.py

@@ -1,6 +1,8 @@
 import ast
 import calendar
 import datetime
+import operator
+from functools import reduce
 
 import requests
 from django.contrib.contenttypes.fields import GenericForeignKey, GenericRelation
@@ -105,7 +107,6 @@ def _gather_data_periodic(self, since, period):
         scale but works for now.
         """
         OFFSET = "2 hours"  # HACK!
-        ctid = ContentType.objects.get_for_model(self).id
 
         c = connections["default"].cursor()
         c.execute(
@@ -117,10 +118,14 @@ def _gather_data_periodic(self, since, period):
                        AND object_id = %s
                        AND timestamp >= %s
                      GROUP BY 1;""",
-            [period, OFFSET, ctid, self.id, since],
+            [period, OFFSET, self.content_type.id, self.id, since],
         )
         return [(calendar.timegm(t.timetuple()), float(m)) for (t, m) in c.fetchall()]
 
+    @property
+    def content_type(self):
+        return ContentType.objects.get_for_model(self)
+
 
 class TracTicketMetric(Metric):
     query = models.TextField()
@@ -249,6 +254,17 @@ def link(self):
         return self.urljoin(self.jenkins_root_url, "job", self.build_name)
 
 
+class DatumQuerySet(models.QuerySet):
+    def metrics(self, *metrics):
+        """
+        Return only the data from the given metrics.
+        """
+        if not metrics:
+            return self.none()
+        qobjs = [models.Q(content_type=m.content_type, object_id=m.pk) for m in metrics]
+        return self.filter(reduce(operator.or_, qobjs))
+
+
 class Datum(models.Model):
     metric = GenericForeignKey()
     content_type = models.ForeignKey(
@@ -258,6 +274,8 @@ class Datum(models.Model):
     timestamp = models.DateTimeField(default=datetime.datetime.now)
     measurement = models.BigIntegerField()
 
+    objects = DatumQuerySet.as_manager()
+
     class Meta:
         ordering = ["-timestamp"]
         get_latest_by = "timestamp"

dashboard/views.py

@@ -1,6 +1,6 @@
 import datetime
+import operator
 
-from django.contrib.contenttypes.models import ContentType
 from django.core.cache import cache
 from django.forms.models import model_to_dict
 from django.http.response import Http404, JsonResponse
@@ -22,25 +22,21 @@ def index(request):
             metrics.extend(
                 MC.objects.filter(show_on_dashboard=True).select_related("category")
             )
-
-        content_types = ContentType.objects.get_for_models(*metrics)
-        datum_queryset = Datum.objects.none()
-        for metric, content_type in content_types.items():
-            datum_queryset = datum_queryset.union(
-                Datum.objects.filter(
-                    content_type_id=content_type.id, object_id=metric.id
-                ).order_by("-timestamp")[0:1]
-            )
-
-        latest_datums = {
-            (datum.object_id, datum.content_type_id): datum for datum in datum_queryset
+        metrics = sorted(metrics, key=operator.attrgetter("display_position"))
+
+        metric_latest_querysets = [
+            Datum.objects.metrics(metric).order_by("-timestamp")[0:1]
+            for metric in metrics
+        ]
+        data_latest = Datum.objects.none().union(*metric_latest_querysets)
+        latest_by_metric = {
+            (datum.content_type_id, datum.object_id): datum for datum in data_latest
         }
 
         data = []
-        for metric, content_type in content_types.items():
-            if latest := latest_datums.get((metric.id, content_type.id)):
-                data.append({"metric": metric, "latest": latest})
-        data = sorted(data, key=lambda elem: elem["metric"].display_position)
+        for metric in metrics:
+            latest = latest_by_metric.get((metric.content_type.pk, metric.pk))
+            data.append({"metric": metric, "latest": latest})
         cache.set(key, data, 60 * 60, version=generation)
 
     return render(request, "dashboard/index.html", {"data": data})