Refine `@DocumentReference` documentation about `self._id` references

[rfelgent]

Hello,

according to documentation this works

@Document
class Book {

  @Id
  ObjectId id;
  String title;
  ObjectId publisherId;
}

@Document
class Publisher {

  @Id
  ObjectId id;
  String acronym;
  String name;

  @ReadOnlyProperty
  @DocumentReference(lookup="{'publisherId':?#{#self._id} }")
  List<Book> books;
}

I tried to change the data type from ObjectId to String and it does not work.

@Document
class Book {

  @Id
  String id;
  String title;
  String publisherId;
}

@Document
class Publisher {

  @Id
  String id;
  String acronym;
  String name;

  @ReadOnlyProperty
  @DocumentReference(lookup="{'publisherId':?#{#self._id} }")
  List<Book> books;
}

What am I missing ?

[mp911de]

What is the data type of your identifiers in the database? Using @Id with String identifiers can cause that the identifier is converted to String in the domain model but can be an ObjectId in the database and therefore, the query {'publisherId':?#{#self._id} } would use a String in the input while the database Id is an ObjectId and therefore yield no matches.

Have you tried using @MongoId instead of @Id

[rfelgent]

Quick response: In my current project,I modelled neither @Id nor @MongoId and fallback to the logic "repository logic should honor a field named id". The given example in my first comment was more or less an illustration of my problem.

Indeed, I could try with `@MongoId. Is there another possibility to influence the corresponding logic without using annotations (without touching the model) ?

[mp911de]

Just to confirm, my assumption holds true:

{
  "_id": {
    "$oid": "67d98819f948d44f0a88565e"
  },
  "title": "title 0",
  "publisherId": "67d98819f948d44f0a88565d",
  "_class": "com.example.demo.Book"
}

When using String Id's, then the object is actually using an ObjectId. As you can see, the automatic conversion only happens for the identifier while publisherId uses a plain string.

The reference lookup operates on the result Document to build the @DocumentReference lookup filter which contains an object Id for _id. We deliberately do not use the materialized entity as entities are under construction and especially when creating entities via constructor, there's no entity object available yet.

To make this work, actually Book.publisherId must be stored as ObjectId. Annotating this property with @Field(targetType = FieldType.OBJECT_ID) makes it work with String-typed properties.