C# class to BSON roundtrip

Question

I am attempting to use MongoDB to store some classes that I habe in an application.

The code I use is

string json = JsonConvert.SerializeObject(item);
BsonDocument document = BsonDocument.Parse(json);
...
await collection.InsertOneAsync(document);

Which seems to work fine. When I retrieve the documents from MongoDB I get a BsonDocument which has an "_id" attribute added.

How do I get back to an instance of the original class (which does not have the _id attribute)

Answer 1

MongoDB automatically adds in the object ID (_id) attribute to serve as the primary key. This happens the moment you do any sort of insertion into MongoDB (no matter if you import a JSON, BSON, CSV, or whatever)

http://docs.mongodb.org/manual/reference/object-id/

If you want to get around this issue in your C# application, one option would be to export from MongoDB to CSV, and then use a tool to import the CSV into a JSON, while selectively avoiding the _id. This seems a bit complex unnecessarily though.

Perhaps you can just keep the _id attribute and make it an optional parameter in your original C# class? Can you explain why the presence of the _id parameter an issue to begin with?

Answer 2

You can also remove the _id field from the BsonDocument after you have read it from the database:

var document = collection.Find(new BsonDocument()).FirstAsync();
document.Remove("_id");

This only removes the _id from the BsonDocument in memory (the value of the document variable), not from the document stored in the database.