Setup and Non Setup Update error

Question

I am trying to update a custom object records based on standard User object.

Here are my conditions:

1. If a New user is added then the Status of the ERP_SalesAgent__c (custom object)is set to Add.
2. If Any Changes Happened on User in step1 then the ERP_SalesAgent__c's status is set to "Change".
3. If User in Step1 is inactive then Status is set to "Delete". while testing the 3rd condition i am facing Mixed DML Operation exception. to avoid this I ran the update on User in Future class but no luck.

Here is my code:

public with sharing class InternalSalesAgent {
public static void ErpInternalSalesAgentRecord( list< User> newUsers )
    {
        set<Id>usersIdsSet = new set<Id>();
        set<string> erpCompanyNumbers = new set<String>();
        list<ERPSalesAgent__c> salesAgents = new list<ERPSalesAgent__c>();
        map< string, list< ERPSalesAgent__c > > erpRecordsMap = new  map< string, list< ERPSalesAgent__c > >( );
        map<string,User> Usermap = new map<string,User>();
        list<User>updateUsers = new list<User>();
        set<Id>UserIdsUpdate = new set<Id>();
        for(User usr:newUsers)
        {
            usersIdsSet.add(usr.Id);
            erpCompanyNumbers.add(usr.ERP_Company_Number__c);
            //System.debug('The new record is:+' +acc.Name);
        }
        for(ERPSalesAgent__c erpExternalAgents:[select id,User__c, Action__c, ERP_Company_Number__c,ERPSalesAgent_ID__c from ERPSalesAgent__c where User__c IN : usersIdsSet and ERP_Company_Number__c IN: erpCompanyNumbers])
        {
            list< ERPSalesAgent__c> erpagents = erpRecordsMap.get( erpExternalAgents.User__c + erpExternalAgents.ERP_Company_Number__c );
            if ( erpagents == null ) {
                erpagents = new list< ERPSalesAgent__c >( );
                erpRecordsMap.put( erpExternalAgents.User__c + erpExternalAgents.ERP_Company_Number__c , erpagents);
            }
            erpagents.add( erpExternalAgents );
        }
        for(User usrs:[select id,Name,ERP_Company_Number__c,IsSalesAgent__c,SalesAgent_ID__c,IsChange__c,IsActive from User where id IN:usersIdsSet and IsSalesAgent__c = true and ERP_Company_Number__c!=null])
        {

              System.debug('The new record is'+usrs);
               Usermap.put(usrs.id + usrs.ERP_Company_Number__c, usrs);
                if(usrs.SalesAgent_ID__c==null && (!erpRecordsMap.containsKey(usrs.Id + usrs.ERP_Company_Number__c) ||  erpRecordsMap.isEmpty() ) )
                {

                    ERPSalesAgent__c  erpagentsRecord = new ERPSalesAgent__c ();
                    erpagentsRecord.User__c = usrs.Id;
                    erpagentsRecord.Action__c = 'Add';
                    erpagentsRecord.ERP_Company_Number__c = usrs.ERP_Company_Number__c;
                    System.debug('The SalesAgent record is:' +erpagentsRecord.User__c);
                    salesAgents.add(erpagentsRecord);

                }
                else if(usrs.SalesAgent_ID__c!=null && erpRecordsMap.containsKey(usrs.Id + usrs.ERP_Company_Number__c) && usrs.IsChange__c && usrs.IsActive)
                {
                    UserIdsUpdate.add(usrs.Id);
                 for(ERPSalesAgent__c erpagentsRecord1 :erpRecordsMap.get(usrs.Id + usrs.ERP_Company_Number__c))
                 {
                    erpagentsRecord1.Action__c = 'Change';
                     salesAgents.add(erpagentsRecord1);
                 }
                 //comment them   
             /* User updtUsers = new User(Id=usrs.Id,IsChange__c = false);
              updateUsers.add( updtUsers );
                    UserIdsUpdate.add(usrs.Id);*/

                }
                else if(usrs.SalesAgent_ID__c!=null && erpRecordsMap.containsKey(usrs.Id + usrs.ERP_Company_Number__c) && !usrs.IsActive)
                {
                    UserIdsUpdate.add(usrs.Id);
                    System.debug('The Inactive SalesAgent is:'+usrs);
                 for(ERPSalesAgent__c erpagentsRecord2 :erpRecordsMap.get(usrs.Id + usrs.ERP_Company_Number__c))
                 {
                    erpagentsRecord2.Action__c = 'Delete';
                     salesAgents.add(erpagentsRecord2);
                 }
                   //comment them 
              /*User updtUsers1 = new User(Id=usrs.Id,IsChange__c = false);
              updateUsers.add( updtUsers1 );
                    UserIdsUpdate.add(usrs.Id);*/

                }



        }

        if(salesAgents.isEmpty())
            return;
        update salesAgents;
      //  if(UserIdsUpdate.isEmpty())
          //  return;
       UserHandler.UpdateUsers(UserIdsUpdate);
       // isRecursive = false;
       // update updateUsers;
        //isRecursive = true;

    }


}

public with sharing class UserHandler {
    @future
    public static void UpdateUsers(set<Id>UserIdsUpdate){
     List<User>newUsers = new List<User>();
     for(Id userIds:UserIdsUpdate)
     {

         newUsers.add(new User(Id = userIds,IsChange__c=false));
     }
        update newUsers;
}
}

Answer 1

I figured out the solution. Here it is:

You can't perform a DML operation on a setup and a non-setup operation in a single class or trigger. In the above situation User is a setup object and ERPSalesAgent is a non-setup object.

The solution is I have changed the operation on setup object to where it's now in the normal call and the non-setup operation has been moved into a call to an @future class. That seems to work without any errors.