Add unique constraint to ProgramEnrollment model

lms/djangoapps/program_enrollments/migrations/0003_auto_20190424_1622.py

@@ -0,0 +1,21 @@
+# -*- coding: utf-8 -*-
+# Generated by Django 1.11.20 on 2019-04-24 16:22
+from __future__ import unicode_literals
+
+from django.conf import settings
+from django.db import migrations
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        migrations.swappable_dependency(settings.AUTH_USER_MODEL),
+        ('program_enrollments', '0002_historicalprogramcourseenrollment_programcourseenrollment'),
+    ]
+
+    operations = [
+        migrations.AlterUniqueTogether(
+            name='programenrollment',
+            unique_together=set([('user', 'external_user_key', 'program_uuid', 'curriculum_uuid')]),
+        ),
+    ]

lms/djangoapps/program_enrollments/models.py

@@ -30,6 +30,12 @@ class ProgramEnrollment(TimeStampedModel):  # pylint: disable=model-missing-unic
     class Meta(object):
         app_label = "program_enrollments"
 
+        # A student enrolled in a given (program, curriculum) should always
+        # have a non-null ``user`` or ``external_user_key`` field (or both).
+        unique_together = (
+            ('user', 'external_user_key', 'program_uuid', 'curriculum_uuid'),
+        )
+
     user = models.ForeignKey(
         User,
         null=True,