i create new module called employee transfer via inheritance.

i want to make field store  value from  employees master table . 

when i create many2one field i don't want to store the id of the relation , i want to store other column example i want to passport number  .

I try to follow some example to store the passport number i don't understand the concept

department_id = fields.Many2one(related='employee_id.department_id', \
                    string='Department', type='many2one', relation='hr.department', \
                    readonly=True, store=True)