Query MongoDB Using 'ObjectId'

Question

I have inserted documents into MongoDB without an id. And I want to retrieve them by searching through their MongoDB ObjectId, that has been assigned in default.

Here is my attempt-

var query_id = Query.EQ("_id", "50ed4e7d5baffd13a44d0153");
var entity = dbCollection.FindOne(query_id);
return entity.ToString();

And I get following error-

A first chance exception of type 'System.NullReferenceException' occurred

What is the problem?

Answer 1

You need to create an instance of ObjectId and then query using that instance, otherwise your query compares ObjectIds to string and fails to find matching documents.

This should work:

var query_id = Query.EQ("_id", ObjectId.Parse("50ed4e7d5baffd13a44d0153"));
var entity = dbCollection.FindOne(query_id);
return entity.ToString();

Answer 2

In C# for latest official MongoDB.Driver write this-

var filter_id = Builders<MODEL_NAME>.Filter.Eq("id", ObjectId.Parse("50ed4e7d5baffd13a44d0153"));
var entity = dbCollection.Find(filter).FirstOrDefault();
return entity.ToString();

We can accomplish the same result without converting id from string to ObjectId. But then, we will have to add [BsonRepresentation(BsonType.ObjectId)] before id attribute in the model class.

The code can even be further simplified using lambda expression-

var entity = dbCollection.Find(document => document.id == "50ed4e7d5baffd13a44d0153").FirstOrDefault();
return entity.ToString();

Answer 3

If you're here in 2018 and want copy/paste code that still works or pure string syntax;

    [Fact]
    public async Task QueryUsingObjectId()
    {
        var filter = Builders<CosmosParkingFactory>.Filter.Eq("_id", new ObjectId("5b57516fd16cb04bfc35fcc6"));
        var entity = stocksCollection.Find(filter);
        var stock = await entity.SingleOrDefaultAsync();
        Assert.NotNull(stock);

        var idString = "5b57516fd16cb04bfc35fcc6";
        var stringFilter = "{ _id: ObjectId('" + idString + "') }";
        var entityStringFiltered = stocksCollection.Find(stringFilter);
        var stockStringFiltered = await entityStringFiltered.SingleOrDefaultAsync();
        Assert.NotNull(stockStringFiltered);
    }

Answer 4

The selected answer is correct. For anyone confused by the Query.EQ, here is another way to write a basic update (updates the entire mongodb document):

string mongoDocumentID = "123455666767778";
var query = new QueryDocument("_id", ObjectId.Parse(mongoDocumentID)); 
var update = new UpdateDocument { { "$set", documentToSave } };
mongoCollection.Update(query, update, UpdateFlags.Multi);

The ObjectId object is needed when you want to actually search by object ID, otherwise it is comparing string to objectid type, and it won't match. Mongo is very type-strict in this way, regardless if the field name is correct.