Spring MongoDB data can not fetch @DBRef objects with "find" query

Question

There is an object a classic POJO as below:

@Document
public class MyPojo {
  @DBRef
  @Field("otherPojo")
  private List<OtherPojo> otherPojos;
}

And OtherPojo.java:

public class OtherPojo{
  @Id
  private ObjectId _id;
  private String someOtherFields;
}

I can not cascade-save these, but I'm getting over it by saving DBRefs first and then saving my POJO list, but still when I try to fetch all list or querying some of them with below code:

Query query = new Query( Criteria.where( "myPojo.blabla" ).is( "blabla" ) );
List<MyPojo> resultList = mongoTemplate.find( query, MyPojo.class, "myCollection" );

it returns me with a list of null DBrefs, it counts true. For example: there are saved 10 DBRefs it returns 10 null object in it, but its primitive types and other types that aren't a DBRref are all non-null. How can i handle this ?

I save my objects as below:

for (MyPojo pojo : somePojoList) {
    for (OtherPojo otherPojo : pojo.getOtherPojos()) {
        mongoTemplate.save(otherPojo, "myCollection");
    }
}

// ...

mongoTemplate.insert( myPojoList, "myCollection" );

EDIT: OK, now I know if I do not specify a collection name when I'm saving otherPojos, I can fetch them (thanks to @jmen7070). But I have to write myCollection there because I'm dropping and re-creating them always. That's a use-case. So how can I say "find method to use same collection to fetch DBRefs"?

Can you give more detail about how do you save objects in data base? — jmen7070
– jmen7070, Commented Apr 29, 2014 at 11:07
I’m also facing same issue, please let me what is the solution. — Ravi S.
– Ravi S., Commented Sep 16, 2021 at 7:48

jmen7070 · Accepted Answer · 2014-04-30 08:07:35Z

2

As you can see from docs:

The mapping framework does not handle cascading saves. If you change an Account object that is referenced by a Person object, you must save the Account object separately. Calling save on the Person object will not automatically save the Account objects in the property accounts.

So, first, you have to save each object of the otherPojos list. After that you can save MyPojo instance:

MyPojo pojo = new MyPojo();
OtherPojo otherPojo = new OtherPojo();
OtherPojo otherPojo1 = new OtherPojo();

pojo.setOtherPojos(Arrays.asList(otherPojo, otherPojo1));

mongoTemplate.save(otherPojo);
mongoTemplate.save(otherPojo1);

mongoTemplate.save(pojo);

UPDATED: You saved a objects:

for( MyPojo pojo : somePojoList ){
            for( OtherPojo otherPojo : pojo.getOtherPojos() ){
                mongoTemplate.save( otherPojo,collectionname );
            }
        }

All otherPojo objects will be saved in collection with name "collectionName"..

but your myPojo objects have a $ref to an otherPojo collection..

"otherPojo" : [ 
        {
            "$ref" : "otherPojo",
            "$id" : ObjectId("535f9100ad52e59815755cef")
        }, 
        {
            "$ref" : "otherPojo",
            "$id" : ObjectId("535f9101ad52e59815755cf0")
        }
    ]

So, "collectionname" variable

 mongoTemplate.save( otherPojo,collectionname );

must be "otherPojo".

To avoid confusion, I propose to specify a collection for saving OtherPojo objects with @Doucument annotation:

@Document(collection="otherPojos")
public class OtherPojo{

@Id
private ObjectId _id;
private String someOtherFields;

}

And to save otherPojo objects by using a overloaded save() method of mongoTemplate

mongoTemplate.save( otherPojo );

In this way , you will have a valid $ref for myPojo documents

UPDATE 2:

In this case, you want to store parents and childs object in same collection.

To achieve this you can use this approach

edited Apr 30, 2014 at 8:07

answered Apr 29, 2014 at 9:32

jmen7070

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7 Comments

Sercan Ozdemir Over a year ago

no !! you misunderstood, read again that part:

ok, now i can not cascade save these but i get over it by saving DBRefs first and then saving my pojo list, but still when i try to fetch all list or querying some of them with below code:

Sercan Ozdemir Over a year ago

ok, i followed your suggestions, i succesfuly fetch them now, but i have to specify a collection name to it.. I can give it a different collection name does it solve ?

jmen7070 Over a year ago

I don't understand your last question, sorry) What is your goal? Which collections do you want to save objects?

Sercan Ozdemir Over a year ago

Ok i have a couple of collections, and iam always dropping re-creating them. This is a use-case, and i cant change that. So, now i have to specify same collection name for "MyPojo" and "OtherPojo" and also when i use find method, im passing this collection name in it. I'm editing question please see there.

jmen7070 Over a year ago

If you will store "MyPojo" and "OtherPojo" objects in same collection, maybe, you should not to use DBRef.. You can store Other Pojos embedded in MyPojo documents.

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