How to solve "Expected element name to be '_t', not 'number'."

Question

I have a mongo model like this:

class ObjectA {
        [BsonId(IdGenerator = typeof(BsonObjectIdGenerator))]
        public BsonObjectId Id;

        [BsonElement("number")]
        public int Number { get; set; }

        [BsonElement("b")]
        public List<ObjectB> objectB { get; set; }
}

class ObjectB {
        [BsonElement("someProperty")]
        public string SomeProperty { get; set; }
}

My problem is when I aggregate the collection with {$unwind:objectB}. The result documencts have a unique object on the property objectB (not a list). So the cast failes with the exception:

An error occurred while deserializing the ObjectB property of class ObjectA: Expected element name to be '_t', not 'number'.

Do I have to create a new model for this or is there a easier way to solve it?

Answer 1

You could also choose to work with BsonDocument directly (but that is not strongly typed and more cumbersome to work with), e.g. (I'm using the simple Posts/Tags example here)

var aggregationResults = db.GetCollection("Posts").Aggregate().ResultDocuments;
foreach (var document in aggregationResults)
{
    var tag = document.GetValue("Tags").AsString;
}

Unlike the normal query and projection operators, the aggregation framework may change the structure of your document. As you already pointed out, $unwind transforms a document that contains an array into a number of documents that each have a single value of the same name.

Another approach this is to indeed create a new type for this, so

class Post {
    public List<string> Tags { get; set; }
    ...

would become

class PostAggregationResult {
    public string Tags { get; set; }
    ...

That is very easy to work with, but if you have very various aggregation queries, you need a large number of classes which can be annoying.