Projection of mongodb BsonId using C# .NET driver 2.2.4

Question

I have the following structure:

    public abstract class DbEntity
{
    [BsonId]
    public ObjectId Id { get; set; }

    [BsonDateTimeOptions(Kind = DateTimeKind.Utc)]
    public DateTime CreatedAt { get; set; }

    [BsonDateTimeOptions(Kind = DateTimeKind.Utc)]
    public DateTime ModifiedAt { get; set; }

    [BsonDefaultValue(EntityState.Active)]        
    public EntityState State { get; set; }

    [BsonIgnoreIfNull]
    [BsonDateTimeOptions(Kind = DateTimeKind.Utc)]
    public DateTime? DeletedOn { get; set; }
}


    public class Post : DbEntity
{
    public ObjectId UserId { get; set; }

    public string SourceUrl { get; set; }

    public Guid? PictureId { get; set; }

    public PostType Type { get; set; }

    public string Caption { get; set; }

    public GeoJsonPoint<GeoJson2DGeographicCoordinates> Location { get; set; }

    public string LocationName { get; set; }

    public List<ObjectId> TaggedFriends { get; set; }

    public List<string> Tags { get; set; }

    public List<string> GearTags { get; set; }

    public Activity Activity { get; set; }

    public int LikesAmount { get; set; }

    public int CommentsAmount { get; set; }

    public List<SharingTarget> Shared { get; set; }

    public List<string> SearchTokens
    {
        get
        {
            var result = new List<string>();
            result.AddRange(Caption.GetTokens());
            result.AddRange(Tags);
            result.AddRange(GearTags);
            result.AddRange(LocationName.GetTokens());
            return result;
        }
        set { }
    }
}

        private class AggregatedPost : Followings
    {
        public AggregatedPost(ObjectId ownerId) : base(ownerId)
        {
        }

        public List<Post> Posts { get; set; }
    }

    private class UnwindPost : Followings
    {
        public UnwindPost(ObjectId ownerId) : base(ownerId)
        {
        }

        public Post Posts { get; set; }
    }

I run the query where as a projection I use an Expression <Func<UnwindPost,Post>>

                          var query = await
                followingsCollection.Aggregate()
                .Match(Builders<Followings>.Filter.Where(v => v.Id == userId))
                .Unwind<Followings>(new ExpressionFieldDefinition<Followings>(LocalFollowingFieldName))
                .Lookup(postCollection, new ExpressionFieldDefinition<Followings>(LocalFollowingFieldName), new ExpressionFieldDefinition<Post>(ForeignPostFieldName),
                new ExpressionFieldDefinition<AggregatedPost>(AggregatedPostAliasFieldName))
                        .Unwind<UnwindPost>(new ExpressionFieldDefinition<AggregatedPost>(UnwindPostFieldName))
                            .Limit(pageInfo.Take)
                            .Skip(pageInfo.Skip)
                            .Project(v => new Post
                            {
                                Id = v.Posts.Id,
                                UserId = v.Posts.UserId,
                                CreatedAt = v.Posts.CreatedAt,
                                SourceUrl = v.Posts.SourceUrl,
                                Type = v.Posts.Type,
                                Caption = v.Posts.Caption,
                                Activity = v.Posts.Activity,
                                LocationName = v.Posts.LocationName,
                                LikesAmount = v.Posts.LikesAmount,
                                CommentsAmount = v.Posts.CommentsAmount,
                                PictureId = v.Posts.PictureId
                            }).ToListAsync();

As a result I have an exception that says Element 'Id' does not match any field or property of class. If I run this query on mongodb, it returns the next record:

Id: ObjectId("57d91c5393846c3c14792522")
 UserId: ObjectId("57d91c5293846c3c14792512")
 CreatedAt: 09/14/2016 12:45:55 PM (+0300)
 SourceUrl: "test url"
 Type: 0
 Caption: "Test1"
 Activity: NULL
 LocationName: "TestLocationName"
 LikesAmount: 0
 CommentsAmount: 0
 PictureId: NULL

As you may see, there is no field _id here, however id is returned as Id. All my records are stored in mongodb with _id field.

How can I project the record with _id field in mongo syntax that will be mapped correctly to my Post class (preferably using projection that has Expression as an argument)?