Fix serialization error for returning participants

backend/experiment/serializers.py

@@ -1,5 +1,5 @@
 from random import shuffle
-from typing import Optional
+from typing import Optional, Union
 
 from django_markup.markup import formatter
 from django.utils.translation import activate, get_language
@@ -137,7 +137,9 @@ def serialize_block(block_object: Block, language: str = "en") -> dict:
     }
 
 
-def get_upcoming_block(phase: Phase, participant: Participant, times_played: int):
+def get_upcoming_block(
+    phase: Phase, participant: Participant, times_played: int
+) -> dict:
     """return next block with minimum finished sessions for this participant
     if all blocks have been played an equal number of times, return None
 

backend/experiment/tests/test_views.py

@@ -87,6 +87,32 @@ def test_get_experiment(self):
         self.assertIsNotNone(response_json)
         self.assertIn(response_json.get("nextBlock").get("slug"), ("block2", "block3"))
 
+    def test_get_experiment_returning_participant(self):
+        Session.objects.bulk_create(
+            self._get_session_objects(
+                [
+                    self.block1,
+                    self.block2,
+                    self.block3,
+                    self.block4,
+                    self.block1,
+                    self.block2,
+                    self.block3,
+                    self.block4,
+                ]
+            )
+        )
+        response = self.client.get("/experiment/test_series/")
+        self.assertEqual(response.json().get("nextBlock").get("slug"), "block1")
+
+    def _get_session_objects(self, block_list: list[Block]) -> list[Session]:
+        return [
+            Session(
+                block=block, participant=self.participant, finished_at=timezone.now()
+            )
+            for block in block_list
+        ]
+
     def test_experiment_not_found(self):
         # if Experiment does not exist, return 404
         response = self.client.get("/experiment/not_found/")

backend/experiment/views.py

@@ -5,7 +5,7 @@
 from django.utils.translation import gettext_lazy as _, get_language
 from django_markup.markup import formatter
 
-from .models import Block, Experiment, Phase, Feedback, Session
+from .models import Block, Experiment, Feedback, Session
 from section.models import Playlist
 from experiment.serializers import (
     serialize_block,
@@ -14,7 +14,7 @@
 )
 from experiment.rules import BLOCK_RULES
 from experiment.actions.utils import EXPERIMENT_KEY
-from image.serializers import serialize_image
+from participant.models import Participant
 from participant.utils import get_participant
 from theme.serializers import serialize_theme
 
@@ -122,13 +122,29 @@ def get_experiment(
         serialized_phase = serialize_phase(phase, participant, times_played)
         if serialized_phase:
             return JsonResponse(
-                {**serialize_experiment(experiment), **serialized_phase}
+                {
+                    **serialize_experiment(experiment),
+                    **serialized_phase,
+                }
             )
-    # if no phase was found, start from scratch by incrementing times_pleyd
-    times_played += 1
-    request.session[times_played_key] = times_played
-    serialized_phase = serialize_phase(phases[0], participant, times_played)
-    return JsonResponse({**serialize_experiment(experiment), **serialized_phase})
+    # if no phase was found, start from scratch with the minimum session count
+    request.session[times_played_key] = _get_min_session_count(experiment, participant)
+    return get_experiment(request, slug)
+
+
+def _get_min_session_count(experiment: Experiment, participant: Participant) -> int:
+    phases = experiment.phases.all()
+    session_counts = []
+    for phase in phases:
+        session_counts.extend(
+            [
+                Session.objects.exclude(finished_at__isnull=True)
+                .filter(block=block, participant=participant)
+                .count()
+                for block in phase.blocks.all()
+            ]
+        )
+    return min(session_counts)
 
 
 def get_associated_blocks(pk_list):