Schedulable Class - Big Object

Question

I've created a schedulable class that retrieves info from SetupAuditTrail everyday and creates a big object records on a daily basis. The schedulable class works fine. But when I run the test class, it is giving me the following error.

System.UnexpectedException: A callout was unsuccessful because of pending uncommitted work related to a process, flow, or Apex operation. Commit or roll back the work, and then try again.

Here is my test class:

@isTest
private class BM_BO_SetUpAuditTrail_BackUpTest {   
    @isTest(SeeAllData=true)
    public static void testScheduledApex() {
        
        String sch = '0 0 1 * * ? 2022';
        Test.startTest();
        String JobId = System.schedule('Process AuditTrail Backup', sch, new BM_BO_SetUpAuditTrail_BackUp());
        Test.stopTest();
        
        System.assert(JobId != null);                                                      
}
    }

Here is my class itself:

global class BM_BO_SetUpAuditTrail_BackUp implements Schedulable {
    global void execute(SchedulableContext SC) {

        List<SetupAuditTrail> listSAT = [SELECT createdbyid, responsiblenamespaceprefix, action, section, delegateuser, createddate
                                         FROM SetupAuditTrail
                                         WHERE createddate = YESTERDAY ];


        List<BM_BO_SetUpAuditTrail_BackUp__b> listBMSAT = new List<BM_BO_SetUpAuditTrail_BackUp__b>();
        
        try{
            for(SetupAuditTrail SATBD : listSAT){
                BM_BO_SetUpAuditTrail_BackUp__b sat = new BM_BO_SetUpAuditTrail_BackUp__b();
                sat.BM_DateTime__c = SATBD.createddate;
                sat.BM_UserId__c = SATBD.createdbyid;
                sat.BM_SrcNameSpace__c = SATBD.responsiblenamespaceprefix;
                sat.BM_Action__c = SATBD.action;
                sat.BM_Section__c = SATBD.section;
                sat.BM_Delegation__c = SATBD.delegateuser;

                listBMSAT.add(sat);

            }
                database.insertimmediate(listBMSAT);
         
            
        } catch(DMLException e){
            System.debug('The following exception has occurred: ' + e.getMessage());
    }
  }
}

Could you please tell me what I did wrong here?

Answer 1

The reason why it only happens in a Test class is because query on big object is being done as a callout, and a callout is not allowed in the Test class in general.

Per the documentation:

To contain test DML calls to the target big object, use a mocking framework with the batch Apex stub API instead.

Documentation also mentions that it is also not allowed in Apex tests to use mixed DML calls on standard/custom object and Big object (for example, inserting an Account record and then query a Big Object) and they would fail with an error.

See documentation on Building a Mocking Framework with the Stub API