MongoDB POJO and inner/nested classes

[Sebastian Binder]

I've got a question regarding MongoDB Java driver and POJOs/serialization.

I'd like to build up my Java classes as they are represented in the MongoDB collection and then use the (new) POJO feature of MongoDB for fetching data. See: http://mongodb.github.io/mongo-java-driver/3.8/driver-async/getting-started/quick-start-pojo/ and http://mongodb.github.io/mongo-java-driver/3.8/bson/pojos/

Right now this only works if I have two different classes, like

User.class

   import java.util.ArrayList;

   import java.util.List;

   import java.util.UUID;

   import org.bson.codecs.pojo.annotations.BsonId;

   import org.bson.codecs.pojo.annotations.BsonProperty;

   

   public class User {

     @BsonId

     @BsonProperty("_id")

     UUID id;

     private List<UserSession> sessions = new ArrayList<>();

   }

UserSession.class

    import java.time.Instant;

    public class UserSession {

     Instant start;

     Instant end;

   }

But as my collection looks more like the following…

    {

     _id: XYZ,

     sessions: {

       {start: XYZ, end: XYZ},

       {start: XYZ, end: XYZ},

       {start: XYZ, end: XYZ}

     }

   }

… I'd like to have a class that looks like this:

User.class

   import java.util.ArrayList;

   import java.util.List;

   import java.util.UUID;

   import java.time.Instant;

   import org.bson.codecs.pojo.annotations.BsonId;

   import org.bson.codecs.pojo.annotations.BsonProperty;

   

   public class User {

     @BsonId

     @BsonProperty("_id")

     UUID id;

     private List<Session> sessions = new ArrayList<>();

   

     public class Session {

       Instant start;

       Instant end;

     }

   }

This makes sense as every unique sessions belongs directly to a user and with being a nested class I'd be able to access fields of the parent User object from within the Session object.

The problem is that the Java driver now complains about not having an empty/no arguments constructor for my Session class ("By default all POJOs must include a public or protected, empty, no arguments, constructor.").

My CodecProvider is like following:

    CodecProvider codecProvider = PojoCodecProvider.builder()

       .register(User.class, User.Session.class);

Anyone here having an idea how to solve this issue?

Really appreciate your help!

Thanks a lot!

Side note: The code snippets above are just short examples how my code kinda looks like. They are not the full code I'm using so there might be syntax errors in it. It might be helpful to have a look on my non-resolved thread on Stackoverflow as there has been a few comments for clarification: https://stackoverflow.com/questions/52309025/mongodb-pojo-and-inner-nested-classes

[Wan Bachtiar]

This makes sense as every unique sessions belongs directly to a user and with being a nested class I’d be able to access fields of the parent User object from within the Session object.

Hi Sebastian,

Just to clarify, you’re asking about nesting the Session class inside of User ?
Is there a reason not to make the classes both public ? 

public class User {
    @BsonId
    @BsonProperty("_id")
    UUID id;
    private List<Session> sessions = new ArrayList<>();
}
public class Session {
    Instant start;
    Instant end;
}

Regards,
Wan

​

[Sebastian Binder]

Hi Wan,

yes, you are right. I try to nest the Session class inside of User . If i have two separate classes I cannot do things like  this.User.id 
from within my Session for getting the UUID of the parent User . Having a separate class with a field id in my Session class does not really represent my database layout and that's what POJOs in my opinion are supposed to do. They should represent the model of the database. Furthermore this cannot be handled by standard MongoDB POJO implementation.

Regards,

Sebastian