The validation for department and manager relationships to employee are possibly very different depending on the company.

Can two departments have the same manager?  Can an employee have no manager?  Can an employee in one department be managed by a manager in another department?  I've worked in companies where all three are true.

My hunch: Odoo usually leaves such company specific customizations to users.

But at least there must be validation on the employee's department, is it right? I cannot imagine an employee (previously assigned to a department ) and when I select him, the department field remains empty!!

It seems that the HR application is not yet full fledged!