Can't enumerate over objects with MongoDBRef

Question

I have a collection called Products which I am trying to enumerate using the official mongo-csharp driver. However as soon as I try to enumerate the collection (e.g. with a foreach loop) I get the following error.

"Default constructor not found for type MongoDB.Driver.MongoDBRef"

The entity class looks like this

public partial class Product
{
    public BsonObjectId _id { get; set; }
    public string Name { get; set; }
    public string Description { get; set; }
    public string Url { get; set; }
    public int Price { get; set; }
    public string Country { get; set; }
    public MongoDBRef Merchant { get; set; }
}

The entry in the collection looks like the following

{
    "_id" : ObjectId("4cff739fba63c20301ee5bc5"),
    "Name" : "Product Name",
    "Description" : "Product Description",
    "Url" : "http://mysite/products/product-name",
    "Price" : 1200,
    "Country" : "au",
    "Merchant" : {
        "$ref" : "Merchant",
        "$id" : ObjectId("533981033d565e640d000000")
    }
}

And i'm reading it in like this.

var db = Db.Instance.GetDatabase();
var matches = db.GetCollection<Product>("Product").FindAll();

I don't get the error until I do either of the following.

var l = matches.ToList();

OR

foreach (var p in matches) {
   // Do something
}

I know you put the entity class, but ban you provide an example document or two in your question? It might help in recreating this. — Justin Jenkins, Dec 13 '10 at 07:48

Andrew Orsich · Accepted Answer · 2010-12-16T08:42:28.137

Connect to mongo db using mongovue and check that collection and data exists.
Show code of

var db = Db.Instance.GetDatabase();

Should be something like this:

var server = MongoServer.Create("mongodb://localhost:27019");
var db =  server.GetDatabase("database_name");

and than yous code:

var matches = db.GetCollection<Product>("Product").FindAll();

3.I've checked source of mongo driver for c# and i found following in MongoDBRef

 // default constructor is private and only used for deserialization
    private MongoDBRef() {
    }

So i suggest that in you version of mongo driver guys from mongo c# driver team forgot about default constructor. In any way check please that constructor exist/not exist using reflector. 4. And i am 99% sure that constructor not present at yous version of mongo driver. Because when you start to enumerate some mongo collection mongo driver going to desirealize data and in case if default constructor not found throws error.

Sorry I should have mentioned that the Db.Instance.GetDatabase(); call is a wrapper for the Create() and GetDatabase() calls. I can also confirm that the collection and data exists in mongovue — Alex, Dec 16 '10 at 01:30
I was right, it was bug at mongodb that was fixed yesterday. You can see commit with fix here -> https://github.com/mongodb/mongo-csharp-driver/commit/d8b192797437e4d6e27d9aef8527dcb66961beba — Andrew Orsich, Dec 16 '10 at 16:20

Can't enumerate over objects with MongoDBRef

1 Answers1