a different tack on diffing suspend data

Attempt records now have a 'diffed' boolean field, recording whether
they have any cmi.suspend_data elements that might need to be diffed.
It's set to 'false' when elements with that key are saved.

This makes it much quicker to find attempts that need to be diffed, so
the task should be able to get to work much quicker.

see #133

numbas_lti/migrations/0065_attempt_diffed.py

@@ -0,0 +1,18 @@
+# Generated by Django 2.2.13 on 2021-04-07 08:15
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('numbas_lti', '0064_auto_20210303_1050'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='attempt',
+            name='diffed',
+            field=models.BooleanField(default=False),
+        ),
+    ]

numbas_lti/models.py

@@ -502,6 +502,7 @@ class Attempt(models.Model):
 
     deleted = models.BooleanField(default=False)
     broken = models.BooleanField(default=False)
+    diffed = models.BooleanField(default=False)
 
     all_data_received = models.BooleanField(default=False)
 
@@ -1107,6 +1108,8 @@ def diff_scormelements(attempt, key='cmi.suspend_data'):
                 e.save()
             last = e
             lastvalue = value
+        attempt.diffed = True
+        attempt.save()
 
 def resolve_dependency_order(deps):
     order = list(deps.keys())

numbas_lti/save_scorm_data.py

@@ -15,6 +15,7 @@ def save_scorm_data(attempt,batches):
     unsaved_elements = []
     question_scores_changed = set()
     with transaction.atomic():
+        needs_diff = False
         for id,elements in batches.items():
             for element in elements:
                 time = timezone.make_aware(datetime.datetime.fromtimestamp(element['time']))
@@ -44,8 +45,14 @@ def save_scorm_data(attempt,batches):
                             unsaved_elements.append(element)
                         else:
                             raise e
+                if element['key'] == 'cmi.suspend_data':
+                    needs_diff = True
             done.append(id)
 
+    if needs_diff:
+        attempt.diffed = False
+        attempts.save(update_fields=('diffed',))
+
     for number in question_scores_changed:
         attempt.update_question_score_info(number)
     return done,unsaved_elements

numbas_lti/tasks.py

@@ -27,7 +27,7 @@ def attempt_report_outcome(attempt):
 
 @periodic_task(crontab(minute='*'))
 def diff_suspend_data():
-    attempts = Attempt.objects.filter(scormelements__key='cmi.suspend_data',scormelements__diff=None).annotate(n=Count('scormelements')).filter(n__gt=1)
+    attempts = Attempt.objects.filter(diffed=False)
     MAX_TIME = 10
     start = datetime.now()
     if attempts.exists():