Unable to retrieve subdocument items in mongodb c#

Question

Model structure:

//Main doc model
Public class MainDocumentModel
{
   [BsonId]
   [BsonRepresentation(BsonType.ObjectId)]
   public string Id { get; set; }

   public string Name{ get; set; }
   public List<SubDocuementModel> SubDocsList { get; set; }
}

//sub doc model
Public class SubDocumentModel
{
   public string Id { get; set; }

   [BsonIgnoreIfNull]
   public string MainDocId { get; set; }
   public string Name{ get; set; }

   [BsonIgnoreIfNull]
   public List<SubDocuementModel> SubDocsList { get; set; } 
   .
   .
   .
}

In DB when I insert MainDocuement , its structure looks like:

{
    "_id" : ObjectId("54b9d732bb63bc10ec9bad6f"),
    "Name" : "Name-1",
    "SubDocsList  : [{
        "Id" : "54b9e76dbb63bc1890e8761b",
        "Name" : "Sub-Name-1"
    },....]
}

Now I want to retrieve subdocuments where main doc Id == 54b9d732bb63bc10ec9bad6f :

private void getSubDoc(SubDocumentModel oModel)
{
  _MainDoc = _db.GetCollection<MainDocumentModel>("MainDoc");
  _query =  Query<MainDocumentModel>.Where(e => e.Id == oModel.MainDocId);

  //Here I get the exception
  //Exception: An error occurred while deserializing the SubDocsList property
  of class Data.MainDocumentModel: Element 'Id' does not match any field or
  property of class Data.SubDocumentModel.
  private MainDocumentModel _mainDocModel = _MainDoc.FindOneAs<MainDocumentModel>(_query);

 oModel.SubDocsList =
                        _mainDocModel .SubDocsList .FindAll(
                            x => x.Name.ToLower().Contains("NameFilter"));

 //Pagination 
 oModel.SubDocsList =
                        oModel.SubDocsList .Take(oModel.ItemsPerPage)
                            .Skip(oModel.ItemsPerPage*(oModel.PageNo - 1))
                            .ToList();
}

1. Why I am getting above deserializing exception. What I am doing wrong?

2. Here I am applying the pagination logic to the retrieved c# list. How I can apply pagination itself in mongo query?

It's very easy to apply pagination for main doc as:

mainDocCursor = MainDoc .Find(_query).SetSortOrder(SortBy.Ascending("Name")).SetLimit(oModel.ItemsPerPage).SetSkip(oModel.ItemsPerPage * (oModel.PageNo - 1));

How can I achieve the same pagination for subdocuments?

Answer 1

I solved the exception of deserializing, with following small changes:

In SubDocumentModel:

Public class SubDocumentModel
{
   [BsonId]
   [BsonRepresentation(BsonType.ObjectId)]
   public string Id { get; set; }
   .
   .
   .
}

And when I insert the subdocument, I am saving the Id as object id not as a string (previously I was saving it as a string which was causing the problem):

public void addSubDocument(SubDocumentModeloModel)
{
    _MainDoc = _db.GetCollection<MainDocumentModel>("MainDoc");
    BsonDocument subDocument = new BsonDocument
    {
       {"_id", ObjectId.GenerateNewId()},//Previously it was {"Id", ObjectId.GenerateNewId().ToString()}
       {"Type", oModel.Name}
    };

    _query = Query<MainDocumentModel>.Where(e => e.Id == oModel.MainDocId);
    _updateBuilder = Update.Push("SubDocsList", subDocument);
    _MainDoc.Update(_query, _updateBuilder);
}

However I've still not found a way for pagination of subdocument in mongodb-c#