Datanucleus/mongodb: Persisting map of lists

Question

I'm using datanucleus 3.2.5 / JDO for persisting objects to a MongoDB database.

While trying to persist one map of lists I'm getting the following exception:

RuntimeException: json can't serialize type [list element type here]

Some sample code:

@PersistenceCapable
public class SomeClass {
    private Map<String, List<SomeOtherClass>> myAttribute;
    // ...
}


@PersistenceCapable(embeddedOnly="true")
public class SomeOtherClass {
    private String attribute;
    // ...
}

I could get around this problem annotating the embedded attribute as @Serialized, but I would rather prefer a more elegant way.

Am I missing anything? Is there a better approach to this issue?

Answer 1

Quoting Andy's reply to my question in the DataNucleus forums:

No persistence spec defines any support for a container of a container. You are always recommended to make the inner container (List in your case) a field of an intermediate class.

So there are two approaches here:

Use an intermediate class

By far the most elegant and maintainable solution. Following the example toy code:

@PersistenceCapable
public class SomeClass {
    private Map<String, SomeOtherClassContainer> myAttribute;
    // ...
}

@PersistenceCapable(embeddedOnly="true")
public class SomeClassContainer {
    private List<SomeOtherClass> myAttribute;
    // ...
}

@PersistenceCapable(embeddedOnly="true")
public class SomeOtherClass {
    private String attribute;
    // ...
}

Mark the attribute as @Serializable

Ugly and probably a source of headaches, specially if relying on java default serialization.

@PersistenceCapable
public class SomeClass {
    @Serializable
    private Map<String, List<SomeOtherClass>> myAttribute;
    // ...
}

@PersistenceCapable(embeddedOnly="true")
public class SomeOtherClass implements Serializable {
    private String attribute;
    // ...
}