Limit the utilization of serializable transactions

A user encountered this error when deleting a vm:

self.cursor.execute("UPDATE vms SET deleted = now() WHERE email = %s AND
id = %s", ( email, id))

psycopg2.errors.SerializationFailure: could not serialize access due to
read/write dependencies among transactions

capsulflask/db_model.py

@@ -13,7 +13,7 @@ class DBModel:
   def __init__(self, connection, cursor):
     self.connection = connection
     self.cursor = cursor
-    self.cursor.execute("SET TRANSACTION ISOLATION LEVEL SERIALIZABLE;")
+
 
 
   #     ------    LOGIN     ---------     
@@ -361,6 +361,7 @@ class DBModel:
   def claim_operation(self, operation_id: int, host_id: str) -> bool:
     # have to make a new cursor to set isolation level
     # cursor = self.connection.cursor()
+    self.cursor.execute("SET TRANSACTION ISOLATION LEVEL SERIALIZABLE;")
     self.cursor.execute("""
       BEGIN TRANSACTION;
       UPDATE host_operation SET assignment_status = 'assigned'