Read null as empty set in springdata-cassandra

Question

I use spring-data-cassandra, and have entity like this:

@Table("users")
public class User {

  @Column("permissions")
  @CassandraType(type = DataType.Name.SET, typeArguments = {DataType.Name.TEXT})
  public Set<String> permissions = new HashSet<>(); 
}

In cassandra I have table users with field permissions of type Set. It works fine when I store some values in the set, but when I try to store empty set, it becomes null when I read such entity from the repository.

Is there a way to force spring-data-cassandra to change null to empty HashSet? Or can I somehow add custom reader for this specific property of the entity?

Answer 1

TL;DR;

That's Cassandra's default behavior to return null for empty Collection and Map-typed columns.

Further Read

Cassandra returns null values for lists, sets, and maps, which do not contain any items. This is especially unfortunate when using classes with pre-initialized fields as seen in your question. There's an open ticket (DATACASS-266 - Loading empty collection-typed properties overwrites pre-initialized fields) in the issue tracker - as of now, without comments or votes.

We're not exactly sure whether it's a good idea to skip setting properties or apply some sort of defaulting when dealing with empty (null) collections as this raises follow-up questions what to do when:

• Creating an instance through constructor creation: A value is required in such case. For property access, we could omit to set the property, for constructor creation we must provide a value.
• The pre-initialized collection contains items but the one received from Cassandra is null.
• We assume, the change would be applied, what will happen with already existing code that assumes empty collections default to null.

A possibility to address this behavior could be configuration on MappingCassandraConverter or an extension point to override so users can apply their own empty collection behavior.