Inconsistent nullability of read() in templates: not sure if mapper can skip a document

[pkubowicz]

In spring-data-mongodb 3.1.8 ReactiveMongoTemplate.ReadDocumentCallback#doWith includes code to deal with null:

			T source = reader.read(type, document);
			if (source != null) {
                        // …
			}

			return Mono.empty();

This code suggests that I can write a custom reader that returns null for elements that cannot be mapped and should be skipped.

However, this is not true. org.springframework.data.convert.EntityReader is not annotated with @Nullable, so source can never be null. In Kotlin, the compiler won't allow writing an implementation returning null.

Use case

By default it is impossible to use findAll on a collection that contains one malformed document: mapping exception will crash the operation and no elements will be emitted. I am looking for a way to ignore invalid documents and return remaining ones.

If I am mistaken and there is a way to write a custom mapper that is able to skip documents that cannot be mapped, please let me know.

[mp911de]

While EntityReader should never return null, there are possibilities where a Converter might return null as Spring's Converter may return null.

In Kotlin, the compiler won't allow writing an implementation returning null.

Actually, Kotlin suggests the right thing to do.

I am looking for a way to ignore invalid documents and return remaining ones.

Spring Data actually never intended to support skipping results, it happened more by accident. I'd suggest registering a lifecycle listener that allows you to update the document prior to mapping it.

[pkubowicz]

Thanks for the explanation. Do you plan to remove the null-handling branch from the code then?

[mp911de]

We decided to revisit the mentioned code paths and to remove code paths that allow for null as a result of EntityReader. Thanks for making us aware of this.