fix enrollment

http-api/app/rules/enrollment.py

@@ -63,6 +63,7 @@ def enroll(
     user = enrollment.user
     course = enrollment.course
     tenant_id = tenant['id']
+    lock_hash = md5_hash('%s%s' % (user.id, course.id))
 
     with persistence_layer.transact_writer() as transact:
         transact.put(
@@ -117,19 +118,24 @@ def enroll(
             },
         )
 
+        class DeduplicationConflictError(Exception):
+            def __init__(self, *args):
+                super().__init__('Enrollment already exists')
+
         # Prevents the user from enrolling in the same course again until
         # the deduplication window expires or is removed
+        transact.condition(
+            key=KeyPair('lock', lock_hash),
+            cond_expr='attribute_not_exists(sk)',
+            exc_cls=DeduplicationConflictError,
+        )
+
         if deduplication_window:
-            lock_hash = md5_hash('%s%s' % (user.id, course.id))
             offset_days = deduplication_window['offset_days']
             ttl_expiration = ttl(
                 start_dt=now_ + timedelta(days=course.access_period - offset_days)
             )
 
-            class DeduplicationConflictError(Exception):
-                def __init__(self, *args):
-                    super().__init__('Enrollment already exists')
-
             transact.put(
                 item={
                     'id': 'lock',
@@ -138,8 +144,6 @@ def enroll(
                     'create_date': now_,
                     'ttl': ttl_expiration,
                 },
-                cond_expr='attribute_not_exists(sk)',
-                exc_cls=DeduplicationConflictError,
             )
             transact.put(
                 item={