more elegant way to sort the queryset, WIP #105

src/viralhostrange/viralhostrangedb/utils.py

+def order_queryset_specifically(*, queryset, actual_order: dict):
+    """
+    From https://stackoverflow.com/a/51291817/2144569
+    :param queryset: the queryset of objects to apply a specific order
+    :param actual_order: a dict where key is pk, and value is the rank of the associated object
+    :type dict
+    :return: the queryset annotate such as the order will follow actual_order
+    """
+    from django.db.models import Case, When, Value, IntegerField
+    return queryset.annotate(
+        rank=Case(
+            *[When(pk=pk, then=Value(actual_order[pk])) for pk in actual_order.keys()],
+            default=Value(len(actual_order)),
+            output_field=IntegerField(),
+        ),
+    ).order_by('rank')

src/viralhostrange/viralhostrangedb/views.py

@@ -35,6 +35,7 @@ from viralhostrangedb import forms, business_process, views_api
 from viralhostrangedb import mixins
 from viralhostrangedb import models
 from viralhostrangedb.business_process import MessageImportationObserver
+from viralhostrangedb.utils import order_queryset_specifically
 
 
 def index(request):
@@ -696,16 +697,20 @@ def download_responses(request):
     aggregated_responses = json.loads(response.rendered_content.decode('utf-8'))
     virus = form.cleaned_data["virus"]
     if virus.exists():
-        actual_order = dict((int(o), i) for i, o in enumerate(request.GET.getlist("virus")))
-        virus = sorted(virus, key=lambda o: actual_order[o.pk])
+        virus = order_queryset_specifically(
+            queryset=virus,
+            actual_order=dict((int(o), i) for i, o in enumerate(request.GET.getlist("virus"))),
+        )
     else:
         virus = models.Virus.objects.filter(
             pk__in=aggregated_responses.keys()
         ).order_by('pk')
     host = form.cleaned_data["host"]
     if host.exists():
-        actual_order = dict((int(o), i) for i, o in enumerate(request.GET.getlist("host")))
-        host = sorted(host, key=lambda o: actual_order[o.pk])
+        host = order_queryset_specifically(
+            queryset=host,
+            actual_order=dict((int(o), i) for i, o in enumerate(request.GET.getlist("host"))),
+        )
     else:
         host = models.Host.objects.filter(
             pk__in=itertools.chain(*[d.keys() for d in aggregated_responses.values()])
@@ -725,11 +730,11 @@ def download_responses(request):
 
     # get at which position each host is, mandatory if data are sparse
     col_pos = dict([(pk, i) for i, pk in enumerate(
-        [o.pk for o in host],
+        host.values_list('pk', flat=True),
         start=virus_infection_ratio_shift,
     )])
     row_pos = dict([(pk, i) for i, pk in enumerate(
-        [o.pk for o in virus],
+        virus.values_list('pk', flat=True),
         start=host_infection_ratio_shift,
     )])