Thanks!
I think it depends on what you’d like to do with this data from list — if classBlist, for example, will be a searchable field, then TypeConverter is not the best solution (mainly because of performance issue, as searching on the database level is much faster than extracting and converting this data every time).
If this field should depend on other field (e.g. every time A is deleted, B should be also), then ForeignKey is the best for this, as you can define such a behaviour.
However, if you don’t need this, and your A class is simply POJO (not a database entity), you may use @Embedded annotation. Thanks to this field will be also searchable.