You're storing departments and employees as separate objects, right? If so, then there is nothing to update in Employee when Department is updated. After any update, all SQL queries will work on the new data (like in any database), so if you join two tables, you will get new results.
If you're going to use SQL queries, you can organize your data model like you would do it in a relational database. In your case you can have departmentId field in Employee class and use this field to join tables. If you update Department, there is nothing to update in the Employee, because they are still linked with each other.
You can refer to query example  for better understanding.
I meant, for example we had to change a deptid for a particular department. Then this is changed in the department table. Now, this deptid should be updated in the employee table as well right, for all those employees who had the previous deptid.