At the moment custom container extractors can be used with the ORM mapper, but the corresponding properties must have automatic indexing disabled (@IndexingDependency(reindexOnUpdate = ReindexOnUpdate.NO)). This is because the code translating container extractor path to paths in the ORM metamodel only understands some built-in extractors, and will fail on custom extractors.
We will need something more than the current ContainerExtractor interface in order to make the ORM mapper understand what a custom extractor "means" exactly.
To test it, we can use a custom container type with a corresponding UserCollectionType.
While we're at it, we should probably test automatic indexing for Optional, OptionalInt, arrays, etc. I think we only have tests for List, Map, Set at the moment.