Allow merging cohorts without magic collections

isic/ingest/services/cohort/__init__.py

@@ -91,8 +91,14 @@ def cohort_merge(*, dest_cohort: Cohort, src_cohort: Cohort) -> None:
         ZipUpload.objects.filter(cohort=src_cohort).update(cohort=dest_cohort)
         MetadataFile.objects.filter(cohort=src_cohort).update(cohort=dest_cohort)
 
-        collection_merge_magic_collections(
-            dest_collection=dest_cohort.collection, src_collection=src_cohort.collection
-        )
+        if src_cohort.collection and dest_cohort.collection:
+            collection_merge_magic_collections(
+                dest_collection=dest_cohort.collection, src_collection=src_cohort.collection
+            )
+        elif src_cohort.collection:
+            dest_cohort.collection = src_cohort.collection
+        # no point in repointing the src collection to the dest collection since it's going away
 
         src_cohort.delete()
+        # dest_cohort has to be saved after the delete to avoid a unique constraint violation
+        dest_cohort.save()

isic/ingest/tests/test_merge.py

@@ -52,6 +52,23 @@ def test_merge_cohorts(full_cohort):
     )
 
 
+@pytest.mark.django_db
+def test_merge_cohorts_missing_magic_collections(full_cohort):
+    """Test that merging a cohort into a cohort with no magic collections works."""
+    dest_cohort, src_cohort = full_cohort(), full_cohort()
+    # coerce the copyright license to test the happy path
+    dest_cohort.copyright_license = src_cohort.copyright_license
+    dest_cohort.save()
+    dest_cohort.collection.delete()
+    dest_cohort.refresh_from_db()
+
+    total_cohort_images = set(src_cohort.collection.images.values_list("pk", flat=True))
+
+    cohort_merge(dest_cohort=dest_cohort, src_cohort=src_cohort)
+    dest_cohort.refresh_from_db()
+    assert set(dest_cohort.collection.images.values_list("pk", flat=True)) == total_cohort_images
+
+
 @pytest.mark.django_db
 def test_merge_cohorts_conflicting_fields(full_cohort):
     cohort_a, cohort_b = full_cohort(), full_cohort()