How to save an object with null dbref in mongodb java spring?

Question

I'm looking for a solution to save an object with a null dbref in mongodb java spring framework. Consider the following example:

@Document
public class A {
    @Id
    private String id;
    @DBRef
    private B b;

    public A() {
        this.b = null;
    }

    ...
}

@Document
public class B {
    @Id
    private String id;
}

Now if I instantiate A, i.e. A a = new A(); and save this object to mongodb via repository, i.e. aRepo.save(a). Then, I have the following exception:

org.springframework.data.mapping.model.MappingException: Cannot create a reference to an object with a NULL id.

Is there a way to save an object with a null dbref?

Thanks for your help!

Answer 1

You are using @DBREF, so you need to create object B first to DB. You can do so:

B b = new B();
mongoOperations.save(b);

A a = new A();
a.setB(b)
mongoOperations.save(a);

Answer 2

Dbref encapsultes multi collection data fetching in a FOREGIN KEY kinda-way. If you're using @DBRef on a field it means that you have this entity already stored, so it must have an @Id. If you want to just store data in an object without cross-collection reference just remove the @DBRef annotation.

For example this is how your data looks in mongodb with @DBRef:

{
    "_id" : ObjectId("5bd1e18ee5adfb64cf7edc5c"),
    "b" : {
        "$ref" : "b",
        "$id" : ObjectId("5bd1e1b7e5adfb65f847159d")
    },
    "_class" : "namespace.A"
}

And this is how it looks without @DBRef

{
    "_id" : ObjectId("5bd1e18ee5adfb64cf7edc5c"),
    "b" : {
        "id" : "someid",
        "anotherfield" : "somevalue"
    },
    "_class" : "namespace.A"
}

Here's how to set a dbref field:

B b = bRepository.findById("someid").get();
A a = new A();
a.setB(b);
aRepository.save(a);