how to use TDBLookupComboBox with xDataClient for a foreign key

BIZ TMS XData
4

Actually I see now that the idea is a little bit different. You should add mapping only for departments:

  ADS_Departements.Close;
  FreeAndnil(Departements_List);
  Departements_List :=  DM_DataAccess.ClientXData.List<Tecl_departement>;
  for departement in Departements_List do
     if not Map.IsIdMapped(departement) then
        Map.Add(departement);
  ADS_Departements.SetSourceList(Departements_List);
  ADS_Departements.Open;

Then after this, for customers, what you should do is replace the departments that are already mapped. Something like this:

  ADS_Users.close;
  FreeAndnil(Users_List);
  Users_List  :=  DM_DataAccess.ClientXData.List<Tecl_utilisateur>;
  for User in Users_List do
     if Map.IsIdMapped(User.departement_id) then
       User.departement_id := Tecl_departement(Map.GetById(Tecl_departement, User.departement_id.id));
  ADS_Users.SetSourceList(Users_List);
  ADS_Users.Open;

That should ok. You only need one Map for all objects, and you create it by passing the TMappingExplorer object:

  Map := TObjectMap.Create(TMappingExplorer.Default);