Map value keeps recreating

[VinceCastro]

I have a Map<enum_type, Map<String, my_class>> where the value Map<String, my_class> of any entry I add keeps resetting to a new HashMap.

The DB is created this way:
var db = DBMaker.fileDB(db_path).transactionEnable().make();

whereas the map itself is created this way:
Map<enum_type, Map<String, my_class>> my_map = (Map<enum_type, Map<String, my_class>>) db.hashMap(name).createOrOpen();

What could be the cause of this issue?

[hiSandog]

This may be a serialization/mutability issue rather than MapDB recreating the value by itself. If the nested Map<String, my_class> is a normal HashMap, each update needs to write the modified value back to the outer map, and the value serializer must be able to serialize my_class deterministically. A minimal example showing whether the nested map is mutated after put, whether db.commit() is called, and which serializer is configured would make the behavior much clearer.