DBRef in Java

[Martinus Martinus]

Hi,

I'm wanted to know how to used DBRef in Java driver, so basically I have this kind of documents about 100 of them with same "mchid", but different timestamp :

collection place

{

     "_id" : ObjectId("4b0552e4f0da7d1eb6f126a2"),

     "users" : 10,

     "male" : 5,

     "female" : 5,

     "mchid" : ObjectId("4b0552e4f0da7d1eb6f126a3"),

       "pos" : [50, 40],

       "time" : 1312121212

}

and I want to refer to this document from another collection in the same database, like below :

collection machine

{

     "_id" : ObjectId("4b0552e4f0da7d1eb6f126a5"),

     "machine" : "server",

     "machine_id" : [{"$ref" : "place", "$id" : "mchid"}]

}

Would you be so kindly to give an example from this simple case?

Thanks.

[Marc]

From the documentation:
"Java supports DB references using the DBRef class."
http://www.mongodb.org/display/DOCS/Database+References

You can make a DataBase reference in java like so:

import com.mongodb.Mongo;
import com.mongodb.DB;
import com.mongodb.DBRef;
import org.bson.types.ObjectId;
import com.mongodb.DBObject;

Mongo connection = new Mongo("localhost", 27017);
DB db = connection.getDB("test");
DBRef myDbRef = new DBRef(db, "place", new
ObjectId("4b0552e4f0da7d1eb6f126a2"));
DBObject doc = myDbRef.fetch();
System.out.println(doc);

The above prints the referenced document:

{ "_id" : { "$oid" : "4b0552e4f0da7d1eb6f126a2"} , "users" : 10.0 ,
"male" : 5.0 , "female" : 5.0 , "mchid" : { "$oid" :
"4b0552e4f0da7d1eb6f126a3"} , "pos" : [ 50.0 , 40.0] , "time" :
1.312121212E9}

For further reading, here is a link to the documentation on the DBRef
class:
http://api.mongodb.org/java/current/com/mongodb/DBRef.html

Hopefully the above will help you accomplish what you need to do.
Good Luck!

[Martinus Martinus]

Hi Marc,

Thanks for your example. So, If I do this : DBRef myDbRef = new DBRef(db, "place", new ObjectId("4b0552e4f0da7d1eb6f126a3"));, it will gave me the same result right?

Thanks.

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[Marc]

I want to say, "yes", but I am not quite sure what you mean by, "the
same result". The line:

DBRef myDbRef = new DBRef(db, "place", new
ObjectId("4b0552e4f0da7d1eb6f126a3"));

will create a DBRef object in Java, but it is only a reference. If
you want to return the document that the DBRef object is referencing,
you must use the .fetch() command as I did in my example.