How do I mock external objects for the purpose of testing a batch Apex class?

Question

I have a Batchable Apex class that queries against an external object in the QueryLocator. I need to be able to write a test against the batch apex class to achieve coverage. I've looked this answer and link which uses the concept of using a method to get query results, but does not use a queryLocator, which I need for the batchable class.

My existing sample POC code works fine when executed from anonymous apex, but I can't seem to get test coverage going.

    global class BatchableLoadPersonsPersisted implements Database.Batchable<SObject> {
    
    global BatchableLoadPersonsPersisted() {
        
    }
    
    /**
     * @description gets invoked when the batch job starts
     * @param context contains the job ID
     * @returns the record set as a QueryLocator object that will be batched for execution
     */ 
    global Database.QueryLocator start(Database.BatchableContext context) {
        return Database.getQueryLocator('SELECT DATEBIRTH__c,DisplayUrl,ExternalId,FIRSTNAME__c,Id,LASTNAME__c,PERSNBR__c,SPOUSEPERSNBR__c FROM Persons__x');
    }

    /**
     * @description gets invoked when the batch job executes and operates on one batch of records. Contains or calls the main execution logic for the batch job.
     * @param context contains the job ID
     * @param scope contains the batch of records to process.
     */ 
    global void execute(Database.BatchableContext context, List<Persons__x> scope) {
        
        List<PersistedPerson__c> persistedPersonsList = new List<PersistedPerson__c>();
        for(Persons__x personx : scope)
        {
            
            PersistedPerson__c persistPerson = new PersistedPerson__c();
            persistPerson.LASTNAME__c = personx.LASTNAME__c;
            persistPerson.FIRSTNAME__c = personx.FIRSTNAME__c;
            persistPerson.DATEBIRTH__c = personx.DATEBIRTH__c;
            persistPerson.PERSONNUMBER__c = personx.PERSNBR__c;
            persistedPersonsList.add(persistPerson);
        }

        upsert persistedPersonsList;
    }
    
    /**
     * @description gets invoked when the batch job finishes. Place any clean up code in this method.
     * @param context contains the job ID
     */ 
    global void finish(Database.BatchableContext context) {
        
    }
}

@isTest
private class BatchableLoadPersonsTests {

    static testmethod void test() {

        // Create test persons to be upserted
        // by the batch job.
        
        Test.startTest();
        Persons__x[] persList = new List<Persons__x>();
        Persons__x person = new Persons__x();
        person.PERSNBR__c = 1;
        person.LASTNAME__c = 'test';
        person.DATEBIRTH__c = Date.newInstance(1960, 2, 17);
        persList.add(person);
        Database.insertImmediate(persList);

         BatchableLoadPersonsPersisted batchLoad = new BatchableLoadPersonsPersisted();
         batchLoad.start(null);
         batchLoad.execute(null, persList);
         batchLoad.finish(null);
        //Database.executeBatch(batchLoad);
        Test.stopTest();

        // Verify persons updated
        PersistedPerson__c[] accUpdatedList = [SELECT Id, LASTNAME__c, FIRSTNAME__c, PERSONNUMBER__c FROM PersistedPerson__c];
        System.assert(!accUpdatedList.isEmpty());
    }
}

Answer 1

I think your best bet is to remove as much logic from the execute method as possible and call another private method that does the iteration.

This way you get coverage, but don't need to worry about the batchable context.

eg:

global void execute(Database.BatchableContext context, List<Persons__x> scope) {
     performBatchActions(scope);
} 

@TestVisible
private void performBatchActions(Persons__x[] scope){
  List<PersistedPerson__c> persistedPersonsList = new List<PersistedPerson__c>();
  for(Persons__x personx : scope)
  {
      
      PersistedPerson__c persistPerson = new PersistedPerson__c();
      persistPerson.LASTNAME__c = personx.LASTNAME__c;
      persistPerson.FIRSTNAME__c = personx.FIRSTNAME__c;
      persistPerson.DATEBIRTH__c = personx.DATEBIRTH__c;
      persistPerson.PERSONNUMBER__c = personx.PERSNBR__c;
      persistedPersonsList.add(persistPerson);
  }

  upsert persistedPersonsList;
}

Either that or just call the execute method directly with a hand-built list of external objects.

If you want to deploy the class on its own with its own unit test method, you'll need 75% coverage - this should be possible, despite missing a couple of lines.

If this is not possible, just cover what you can and deploy using run all tests.