bluewolf-beyond/selector attempt to de-reference a null object

Question

Our org uses a mix of a variety of open source packages including bluewolf-beyond/selector

This is a nifty package that allows construction of sobject list filters using a builder pattern that is more readable and reusable than classic Apex.

For example (here, in an fflib domain method for Assets.cls):

private List<SObject> getCancelledAssets(Map<Id,Sobject> existingRecords) {

   Select.Filter hasCancelledStatusFilter =
    Select.Field.isEqual(Asset.Status, 'Cancelled');

   if (existingRecords != null) {
    return Select.Field.hasChanged(Asset.Status)
       .andX(hasCancelledStatusFilter)
       .filter(Records,existingRecords);
   }
   else {
     return hasCancelledStatusFilter.filter(Records);
   }
}

When running this, the developer got this stacktrace :

System.NullPointerException
LineNumber:39
Message:Attempt to de-reference a null object
Stacktrace:Class.Select.FieldReference.SchemaFieldReference.getFrom: line 39, column 1
Class.Select.FieldChangedPredicate.evaluate: line 22, column 1
Class.Select.AndPredicate.evaluate: line 51, column 1
...

What could be the reason?

Answer 1

Well, in retrospect, the answer is pretty obvious but as the bluewolf-beyond/selector package was fairly new to the developer in question, the developer got frustrated and wanted to revert to classic Apex...

Basically, the clue in the stacktrace was:

Class.Select.FieldChangedPredicate.evaluate: line 22, column 1

this meant the filter Select.Field.hasChanged(Asset.Status) was being evaluated. And this meant that existingRecords was not null. This in turn implied that we were in a before/after update trigger.

But the test case was after insert - so existingRecords should have been null.

Tracing higher up in the code, the domain method was calling

getCancelledAssets(Map<Id,Sobject> existingRecords)

with an empty map in the onAfterInsert handler so the if (existingRecords != null) was TRUE in the after insert use case!

The fix (as we prefer empty collections over nulls) was simply:

private List<SObject> getCancelledAssets(Map<Id,Sobject> existingRecords) {

   Select.Filter hasCancelledStatusFilter =
    Select.Field.isEqual(Asset.Status, 'Cancelled');

   if (!existingRecords.isEmpty()) {
     return Select.Field.hasChanged(Asset.Status)
        .andX(hasCancelledStatusFilter)
        .filter(Records,existingRecords);
   }
   else {
     return hasCancelledStatusFilter.filter(Records);
   }
}   

Bottom line - don't be afraid to poke into the open source code to understand what it is doing to gain insights into what you are doing wrong.