fix: Add a data migration which removes any existing duplicates in the learning sequences

Django app's user partition group model.
fix: Add a unique constraint on that same model on partition_id/group_id to avoid
future duplicates.

TNL-8314

openedx/core/djangoapps/content/learning_sequences/migrations/0014_remove_user_partition_group_duplicates.py

@@ -0,0 +1,47 @@
+# Generated by Django 2.2.24 on 2021-07-07 18:34
+
+from django.db import migrations
+from django.db.models import Count, Min
+
+
+class Migration(migrations.Migration):
+
+    def remove_user_partition_group_duplicates(apps, schema_editor):
+        UserPartitionGroup = apps.get_model('learning_sequences', 'UserPartitionGroup')
+
+        upg_duplicates = (
+            UserPartitionGroup.objects.values("partition_id", "group_id")
+            .annotate(first_entry_id=Min("id"), num_entries=Count("id"))
+            .filter(num_entries__gt=1)
+        )
+
+        for duplicate in upg_duplicates:
+
+            # Repoint the course sections and section sequences which have the duplicate user partition
+            # group entries to the single remaining non-duplicate user partition group.
+            upg_to_keep = UserPartitionGroup.objects.filter(id=duplicate["first_entry_id"])[:1].get()
+            upgs_to_remove = (
+                UserPartitionGroup.objects
+                .filter(partition_id=duplicate["partition_id"], group_id=duplicate["group_id"])
+                .exclude(id=duplicate["first_entry_id"])
+            )
+            for one_duplicate in upgs_to_remove:
+                sections_to_change = one_duplicate.coursesection_set.all()
+                for section in sections_to_change:
+                    section.user_partition_groups.remove(one_duplicate)
+                    section.user_partition_groups.add(upg_to_keep)
+                sequences_to_change = one_duplicate.coursesectionsequence_set.all()
+                for sequence in sequences_to_change:
+                    sequence.user_partition_groups.remove(one_duplicate)
+                    sequence.user_partition_groups.add(upg_to_keep)
+
+            # Remove the duplicate user partition groups.
+            upgs_to_remove.delete()
+
+    dependencies = [
+        ('learning_sequences', '0013_through_model_for_user_partition_groups_1'),
+    ]
+
+    operations = [
+        migrations.RunPython(code=remove_user_partition_group_duplicates, reverse_code=migrations.RunPython.noop)
+    ]

openedx/core/djangoapps/content/learning_sequences/migrations/0015_add_user_partition_group_unique_constraint.py

@@ -0,0 +1,17 @@
+# Generated by Django 2.2.24 on 2021-07-07 19:32
+
+from django.db import migrations
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('learning_sequences', '0014_remove_user_partition_group_duplicates'),
+    ]
+
+    operations = [
+        migrations.AlterUniqueTogether(
+            name='userpartitiongroup',
+            unique_together={('partition_id', 'group_id')},
+        ),
+    ]

openedx/core/djangoapps/content/learning_sequences/models.py

@@ -180,6 +180,9 @@ class UserPartitionGroup(models.Model):
     group_id = models.BigIntegerField(null=False)
 
     class Meta:
+        unique_together = [
+            ['partition_id', 'group_id'],
+        ]
         indexes = [
             models.Index(fields=['partition_id', 'group_id']),
         ]