6

I have a class hierarchy designed for store user notifications:

@Document
public class Notification<T> {
   @Id
   private String id;
   @DBRef
   private T tag;
   ...
}

@Document
public class NotificationA extends Notification<WrappedA> {
}

@Document
public class NotificationB extends Notification<WrappedB> {
}

    ...

This is useful for returning polymorphic arrays, allowing me to store any kind of data in the "tag" field. The problem starts when the wrapped objects contains @DBRef fields:

@Document
public class WrappedA {
   @Id
   private String id;
   @DBRef
   private JetAnotherClass referenced;
   ...
}

Queries on the fields of "tag" works fine:

db.NotificationA.find( {"tag.$id": ObjectId("507b9902...32a")} )

But I need to query on the fields of JetAnotherClass (two levels of @DBRef fields). I've tried with dot notation and also with subobjects but it returns null:

Dot notation: 

db.NotificationA.findOne( {"tag.$referenced.$id": ObjectId("508a7701...29f")} )

Subobjects: 

db.NotificationA.findOne( {"tag.$referenced": { "_id": ObjectId("508a7701...29f") }} )

Any help? Thanks in advance!

Roi
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1 Answers1

7

Since you look like you are only querying by _id I believe you can do:

db.NotificationA.findOne({"tag.$id": ObjectId("blah")});

However:

But I need to query on the fields of JetAnotherClass (two levels of @DBRef fields).

DBRefs are not JOINs, they are merely a self describing _id in the event that you do not know the linking collection it will create a helper object so you don't have to code this yourself on the client side.

You can find more on DBRefs here: http://docs.mongodb.org/manual/applications/database-references/

Basically you can query the sub fields within the DBRef from the same document, i.e.: DBRef.$_id but you cannot, server-side, resolve that DBRef and query on the resulting fields.

Sammaye
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  • Ok, it seems I've made a conceptual error. Is there a way to solve this from Java client? – Roi Nov 13 '12 at 15:10
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    @Roi The only way is to resolve the JOINs client side manually, it is pretty standard across all platforms: http://stackoverflow.com/questions/4067197/mongodb-and-joins – Sammaye Nov 13 '12 at 15:25
  • Starting on Mongo 3.2 you can use `$lookup`, more information here: https://stackoverflow.com/a/39476690/1195507 – rvazquezglez Jan 31 '19 at 01:25