department_id = fields.Many2one('hr.department', string='Dział', required=True, track_visibility='onchange', store=True)

employee_id = fields.Many2many('hr.employee', 'department_id', index=True)

 @api.depends('department_id')

@api.onchange('department_id')

def _compute_employees(self):

emp_data = self.env['hr.employee'].browse([('department_id','in',self.department_id.id)])

for department in self:

department.employee_id = emp_data    

Don't work...