Use of JDO converters to convert a String to a Long is not supported, and is minority interest. Get the code if you require such things

Here's a bug report: https://github.com/datanucleus/datanucleus-mongodb/issues/55

Adding 

```

Looks totally normal to me since you selected to have Availability stored as its own type, hence it has a reference to the object "id" that it relates to.
Obviously you could store them as embedded if you wanted (see the docs).
Or if you expected to see a "DBRef" stored then you can contribute to https://github.com/datanucleus/datanucleus-mongodb/issues/6

When persisting this model with datanucleus-mongodb 

@PersistenceCapable
public class Room implements Serializable {
  @PrimaryKey
  @Persistent(valueStrategy= IdGeneratorStrategy.IDENTITY)
  private String uid;
  private List<Availability> availabilities;
}

This Room model is stored as this document:

{

  "_id": {

    "$oid": "5fe619bde06a02dd8d35a93d"

  },

  "roomName": "Sample Room Name",

  "isPublished": false,

  "availabilities": [

    "5fe619bde06a02dd8d35a93e[OID]com.mycompany.models.Availability",

    "5fe619bde06a02dd8d35a93f[OID]com.mycompany.models.Availability"

  ]

}

Is it normal that the array items is stored like this? It does not look like a normal MongoDB document. 

The Availability model:

@PersistenceCapable
public class Availability implements Serializable {

  private String day;
  @Convert(LocalTimeConverter.class)
  private String opens;
  @Convert(LocalTimeConverter.class)
  private String closes;
}

And the Availability as such: 

{

  "_id": {

    "$oid": "5fe619bde06a02dd8d35a93e"

  },

  "opens": "07:00",

  "day": "SUNDAY",

  "closes": "11:00"

}

Although having a converter:

public class LocalTimeConverter implements AttributeConverter<String, Long> {
    @Override
    public Long convertToDatastore(String value) {
        LocalTime localTime = LocalTime.parse(value);
        return localTime.getLong(ChronoField.NANO_OF_DAY);
    }

    @Override
    public String convertToAttribute(Long value) {
        LocalTime localTime = LocalTime.ofNanoOfDay(value);
        return localTime.toString();
    }
}

I've tried debugging this, and from what I find, the converter is not even called despite annotated properly.