How can I validate DBRefs in a MongoDB collection?

Question

Assuming I've got a MongoDB instance with 2 collections - places and people.

A typical places document looks like:

{
    "_id": "someID"
    "name": "Broadway Center"
    "url": "bc.example.net"
}

And a people document looks like:

{
    "name": "Erin"
    "place": DBRef("places", "someID")
    "url":  "bc.example.net/Erin"
}

Is there any way to validate the places DBRef of every document in the people collection?

Answer 1

There's no official/built-in method to test the validity of DBRefs, so the validation must be performed manually.

I wrote a small script - validateDBRefs.js:

var returnIdFunc = function(doc) { return doc._id; };

var allPlaceIds  = db.places.find({}, {_id: 1} ).map(returnIdFunc);

var peopleWithInvalidRefs = db.people.find({"place.$id": {$nin: allPlaceIds}}).map(returnIdFunc);

print("Found the following documents with invalid DBRefs");
var length = peopleWithInvalidRefs.length;
for (var i = 0; i < length; i++) {
    print(peopleWithInvalidRefs[i]);
}

That when run with:

mongo DB_NAME validateDBRefs.js

Will output:

Found the following documents with invalid DBRefs

513c4c25589446268f62f487

513c4c26589446268f62f48a

Answer 2

you could add a stored function for that. please note that the mongo documentation discourages the use of stored functions. You can read about it here

In essence you create a function:

db.system.js.save(
   {
     _id : "myAddFunction" ,
     value : function (x, y){ return x + y; }
   }
);

and once the function is created you can use it in your where clauses. So you could write a function that checks for the existence of the id in the dbRef.