Question

Been trying to fetch documents from a collection, and the documents contains nested documents.

Getting an error message saying “Expected a nested document representing the serialized form of a Project.Models.MapTags value, but found a value of String instead”

Here is the code

This is the Method getting the collection and does the query.

public List<Maps> GetAllMapsByUserId(ObjectId userId)        
                  {            
                      using (_server.RequestStart(_db))            
                      {                
                          var query = Query.EQ("UserId", userId);
                          MongoCursor<Maps> maps = _db.GetCollection<Maps>"Maps").FindAs<Maps>(query);
                          var list = maps.ToList();

                          return list;            
                      }
                  }

This is the class which contains the object

namespace Project.Models 
{    

public class Maps    {        

    [BsonId]        
    public ObjectId Id { get; set; }
    public ObjectId UserId { get; set; }
    public string MapName { get; set; }
    public string Description { get; set; }
    public BsonBoolean PublicMap { get; set; }
    public DateTime Created { get; set; }

    [BsonIgnoreIfNull]
    public List<MapTags> Tags { get; set; }

    [BsonIgnoreIfNull]
    public List<MapVotes> Votes { get; set; }

    }

public class MapVotes
{
    public ObjectId VoterId { get; set; }
    public int VoteValue { get; set; }
}

public class MapTags
{
    public string Tag { get; set; }
}

}

Hope that someone has an idea/tip or similar, been searching and testing for a long time and haven’t found a solution.

Update :

Here is an example of data in the database

  { 
    "_id" : ObjectId("4eda2415851e702684bf6392"), 
    "MapName" : "Test", 
    "Description" : "Test", 
    "Created" : ISODate("2011-12-03T13:28:53.698Z"), 
    "PublicMap" : false,
    "UserId" : ObjectId("4e8033a0851e701c7c1e12e1"), 
    "Tags" : [ "Test", "Kalle", "Jonas", "Fredrik" ] 
  }

Answer 1

It looks like the serializer expects a document, but finds a string.

The problem is that you have serialized an array of strings:

{
   "MapName" : "SomeMapName"
   "Tags" : ["tag1", "tag2"] // list of strings
   // ...
}

but you should have

{
   "MapName" : "SomeMapName"
   "Tags" : [ { /* map tag document */ }, { /* map tag document */ }, ...]
   // ...
}

The deserializer expects to find a document, but finds only a string. Now it’s possible that the MapTag class can be essentially serialized using a string and can be reconstructed from a string, but you have to provide a custom serializer for that. It’s pretty straightforward.