I have set up a simple datastore to store a Version and a Build. Here are their classes-

@Entity("Version")
public class Version {
    @Id
    private ObjectId id = new ObjectId();
    public Version() {}
    private String name;
}

@Entity("Build")
public class Build {
    public Build() {}
    @Id
    ObjectId id = new ObjectId();
    @Reference
    Version version;
    String name;    
}

(the fields in the classes having their getters, setters, constructors and toString implemented).

Now I'm trying to simply store the Version first and then try to store a Build and then read all the builds there are present.

Version version = new Version("first");
Version version2 = new Version("second");

VersionDAO.saveVersion(version);
VersionDAO.saveVersion(version2);
VersionDAO.printAllVersions();

Build build = new Build(version, "Hello");
BuildDAO.saveBuild(build);
BuildDAO.getAllBuilds();

VersionDAO.getAllVersions() works fine and gets me the list of versions stored but BuildDAO.getAllBuilds() throws an error saying-

Exception in thread "main" dev.morphia.mapping.MappingException: Could not map trying_morphia.Build with ID: 602f4e9ff760cd5638698273 in database 'Builds'

Caused by: dev.morphia.mapping.MappingException: The reference ({ "$ref" : "Version", "$id" : "602f4e7b96f28d54bd1cc883" }) could not be fetched for trying_morphia.Build.version

There seems to be some reference linking issue that I cant seem to figure out. I have to use the @Reference as this piece of code is a part of a bigger piece and I cant change the whole of it.

The MongoDB Compass shows the value of a Build entry as-

_id: ObjectId("602f4e9ff760cd5638698273")  
className: "trying_morphia.Build"  
version: DBRef(Version, 602f4e7b96f28d54bd1cc883, undefined)  
name:"Hello"  

What am I missing in the reference?

What I've tried: Changing the id of both Version and Build to string.