Hi all, 

i have 2 models , 

Passport and Employees

class PassportManagementModels(models.Model):    
_name = 'om_hrm.passport'    
_description = 'Passport Master'    
_inherit = ['mail.thread']    
_rec_name = "passport_no"
passport_no = fields.Char(string='Passport Number',size=16,required=True)    PassportIssueDate = fields.Date(string='Passport Issue Date', required=True)    PassportExpiryDate =fields.Date(string="Expiry Date",required=True)    PassportNationality =fields.Char(string="Nationality",size=20,required=True)    Passportdob = fields.Date(string="Date of Birth",required=True)  
Passportholdername = fields.Char(string="Holder Full Name",required=True)    Passportpossesionwith = fields.Selection([("admin","Admin"),("staff","Staff"),("other","Other")],string="Possesion With!")    
passport_doc_ids = fields.Many2many('ir.attachment',string='Passport Documents',ondelete='cascade')

_sql_constraints = [        ('key_uniq', 'unique (passport_no)', 'PassportNo must be unique.')    ]

class EmployeesManagementModels(models.Model):    
_name = 'om_hrm.employee'    _description = 'Employee Master'    
_inherit = ['mail.thread']    
_rec_name = "full_Name"
employeeId = fields.Char(string='Employee id',required= True)    
passport_id = fields.Many2one('om_hrm.passport', string='Passport', ondelete='set null',domain=lambda self:[('id','=',self._get_assigned_passport_ids())])  
@api.onchange('passport_id')
def onchange_passport_id(self):        # Refresh the domain for passport_id to show only unassigned passports        
self._get_assigned_passport_ids()

@api.model    
def _get_assigned_passport_ids(self):        # Get a list of all passport IDs already assigned to any employee       
allemployees = self.search([])        
print("Employees list:",allemployees)        
assigned_passport_ids = self.search([]).mapped('passport_id.id')        print("Assigned Passport IDs:", assigned_passport_ids)        
return assigned_passport_ids

i want to see the passports which are not assigned to any employees,like fetching all passport and checking if the employee table contains the id then it shouldnt show in the dropdown list.

Any help is appreciated, thanks.