Problem persisting a TreeSet field

Hello,

I am getting a "java.lang.IllegalArgumentException: Can not set java.util.TreeSet field Message.children to java.util.HashSet" when persisting a Message object with a parent/children relationship:

@ManyToOne
private Message parent;

@OneToMany(mappedBy = "parent")
private TreeSet<Message> children;

Seems that objectdb is converting TreeSet into HashSet? Why? http://www.objectdb.com/java/jpa/entity/types says TreeSet is supported...

Thanks for pointers to the solution! I need the TreeSet to have a well defined traversal order when iterating through the Set.

Best regards, Benjamin

Your definition of field children is invalid in JPA - Only collection interface types (Collection, Set, List, Map) should be used in persistent field definition. ObjectDB doesn't enforce this rule for ordinary persistent fields - but mapped by (inverse) fields are more limited.

The order of elements in a mapped by (inverse) collection field is set by an automatic query. You can define the field as List and use the @OrderBy annotation to refine the automatic query to return ordered elements.

Alternatively, you can use TreeSet with your order as an ordinary persistent field instead of as an inverse field - remove he mappedBy setting, and update the TreeSet explicitly - ObjectDB doesn't need a bidirectional relationship to store collections, and actually avoiding them may improve performance as explained in the manual.

Thank you very much for your professional, super fast support! Highly appreciated! 

Best, Benjamin