C＃AutoMapperMappingException KeyNotFoundException

Question

I am guessing all the info except the exception is extra and barely relevant (and is here for completeness). The question is: What, exactly, is the problem that was encountered based on the exception below?

I am having trouble understanding an exception message. To the best of my understanding, there was an error mapping BsonDocument to ChildItemViewModel . Id , Description , and SequentialOrgId were mapped successfully...and then something happened, though these are the only properties. Seems like it's looking for _id (a property that doesn't exist) in cI (BsonDocument that is being converted to ChildItemViewModel), but I have no idea if that's correct, because it makes no sense.

This is a snippet of code (the mapping) where I BELIEVE this to be happening, but I have no idea since I don't have line numbers.

        foreach (BsonDocument cI in childItems)
        {
            ChildItemViewModel childItem = Mapper.Map<BsonDocument, ChildItemViewModel>(cI);
            itemView.ChildItems.Add(childItem);
        }

This is the Entity Framework View Model (class):

public class ChildItemViewModel
{
    public long Id { get; set; }
    public string Description { get; set; }
    public long SequentialOrgId { get; set; }
}

This is the source:

Edit: I have confirmed the ChildItems field to be the source of the problem. If I clear them with the code below, the problem goes away:

db.items.update(
   { _id: 1245846 },
   { $set:
      {
            "ChildItems":[], 
            "ChildItemsAmount": NumberInt(0)
      }
   }
)

The exception:

 {"message":"AutoMapper.AutoMapperMappingException: 

 Mapping types:
 BsonDocument -> Int64
 MongoDB.Bson.BsonDocument -> System.Int64

 Destination path:
 ChildItemViewModel.Id.Id

 Source value:
 { \"Id\" : NumberLong(1245854), \"Description\" : \"Image of Ext USB drive containing backup files labeled \\\"Kevin SomeLastName Backups\\\"\", \"SequentialOrgId\" : NumberLong(189) } ---> System.Collections.Generic.KeyNotFoundException: Element '_id' not found.
    at MongoDB.Bson.BsonDocument.get_Item(String name)
    at lambda_method(Closure , BsonDocument )
    at AutoMapper.Internal.DelegateBasedResolver`2.Resolve(ResolutionResult source)
    at AutoMapper.NullReferenceExceptionSwallowingResolver.Resolve(ResolutionResult source)
    at System.Linq.Enumerable.Aggregate[TSource,TAccumulate](IEnumerable`1 source, TAccumulate seed, Func`3 func)
    at AutoMapper.Mappers.TypeMapObjectMapperRegistry.PropertyMapMappingStrategy.MapPropertyValue(ResolutionContext context, IMappingEngineRunner mapper, Object mappedObject, PropertyMap propertyMap)
    --- End of inner exception stack trace ---
    at AutoMapper.Mappers.TypeMapObjectMapperRegistry.PropertyMapMappingStrategy.MapPropertyValue(ResolutionContext context, IMappingEngineRunner mapper, Object mappedObject, PropertyMap propertyMap)
    at AutoMapper.Mappers.TypeMapObjectMapperRegistry.PropertyMapMappingStrategy.Map(ResolutionContext context, IMappingEngineRunner mapper)
    at AutoMapper.MappingEngine.AutoMapper.IMappingEngineRunner.Map(ResolutionContext context)
    at AutoMapper.MappingEngine.Map[TSource,TDestination](TSource source)
    at API.Classes.Automapper.ItemViewModelConverter.Convert(ResolutionContext resolution)
    at AutoMapper.Internal.MappingExpression`2.<>c__DisplayClass51_0.<ConvertUsing>b__0(ResolutionContext context)
    at AutoMapper.MappingEngine.AutoMapper.IMappingEngineRunner.Map(ResolutionContext context)
    at AutoMapper.MappingEngine.Map[TSource,TDestination](TSource source)
    at MongoDB.Bson.Serialization.Serializers.ProjectingDeserializer`2.Deserialize(BsonDeserializationContext context, BsonDeserializationArgs args)
    at MongoDB.Bson.Serialization.IBsonSerializerExtensions.Deserialize[TValue](IBsonSerializer`1 serializer, BsonDeserializationContext context)
    at MongoDB.Driver.Core.WireProtocol.Messages.Encoders.BinaryEncoders.ReplyMessageBinaryEncoder`1.ReadMessage()
    at MongoDB.Driver.Core.Connections.BinaryConnection.<ReceiveMessageAsync>d__15.MoveNext()
 --- End of stack trace from previous location where exception was thrown ---
    at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
    at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
    at MongoDB.Driver.Core.WireProtocol.QueryWireProtocol`1.<ExecuteAsync>d__0.MoveNext()
 --- End of stack trace from previous location where exception was thrown ---
    at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
    at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
    at MongoDB.Driver.Core.Servers.ClusterableServer.ServerChannel.<ExecuteProtocolAsync>d__24`1.MoveNext()
 --- End of stack trace from previous location where exception was thrown ---
    at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
    at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
    at MongoDB.Driver.Core.Operations.FindOperation`1.<ExecuteAsync>d__2.MoveNext()
 --- End of stack trace from previous location where exception was thrown ---
    at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
    at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
    at MongoDB.Driver.OperationExecutor.<ExecuteReadOperationAsync>d__0`1.MoveNext()
 --- End of stack trace from previous location where exception was thrown ---
    at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
    at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
    at MongoDB.Driver.MongoCollectionImpl`1.<ExecuteReadOperation>d__35`1.MoveNext()
 --- End of stack trace from previous location where exception was thrown ---
    at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
    at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
    at MongoDB.Driver.IFindFluentExtensions.<SingleOrDefaultAsync>d__f`2.MoveNext()
 --- End of stack trace from previous location where exception was thrown ---
    at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
    at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
    at API.Controllers.ItemsController.<GetItem>d__2.MoveNext()
 --- End of stack trace from previous location where exception was thrown ---
    at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
    at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
    at System.Threading.Tasks.TaskHelpersExtensions.<CastToObject>d__3`1.MoveNext()
 --- End of stack trace from previous location where exception was thrown ---
    at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
    at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
    at System.Web.Http.Controllers.ApiControllerActionInvoker.<InvokeActionAsyncCore>d__0.MoveNext()
 --- End of stack trace from previous location where exception was thrown ---
    at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
    at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
    at System.Web.Http.Filters.ActionFilterAttribute.<CallOnActionExecutedAsync>d__5.MoveNext()
 --- End of stack trace from previous location where exception was thrown ---
    at System.Web.Http.Filters.ActionFilterAttribute.<CallOnActionExecutedAsync>d__5.MoveNext()
 --- End of stack trace from previous location where exception was thrown ---
    at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
    at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
    at System.Web.Http.Filters.ActionFilterAttribute.<ExecuteActionFilterAsyncCore>d__0.MoveNext()
 --- End of stack trace from previous location where exception was thrown ---
    at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
    at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
    at System.Web.Http.Filters.ActionFilterAttribute.<CallOnActionExecutedAsync>d__5.MoveNext()
 --- End of stack trace from previous location where exception was thrown ---
    at System.Web.Http.Filters.ActionFilterAttribute.<CallOnActionExecutedAsync>d__5.MoveNext()
 --- End of stack trace from previous location where exception was thrown ---
    at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
    at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
    at System.Web.Http.Filters.ActionFilterAttribute.<ExecuteActionFilterAsyncCore>d__0.MoveNext()
 --- End of stack trace from previous location where exception was thrown ---
    at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
    at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
    at System.Web.Http.Filters.ActionFilterAttribute.<CallOnActionExecutedAsync>d__5.MoveNext()
 --- End of stack trace from previous location where exception was thrown ---
    at System.Web.Http.Filters.ActionFilterAttribute.<CallOnActionExecutedAsync>d__5.MoveNext()
 --- End of stack trace from previous location where exception was thrown ---
    at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
    at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
    at System.Web.Http.Filters.ActionFilterAttribute.<ExecuteActionFilterAsyncCore>d__0.MoveNext()
 --- End of stack trace from previous location where exception was thrown ---
    at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
    at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
    at System.Web.Http.Filters.ActionFilterAttribute.<CallOnActionExecutedAsync>d__5.MoveNext()
 --- End of stack trace from previous location where exception was thrown ---
    at System.Web.Http.Filters.ActionFilterAttribute.<CallOnActionExecutedAsync>d__5.MoveNext()
 --- End of stack trace from previous location where exception was thrown ---
    at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
    at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
    at System.Web.Http.Filters.ActionFilterAttribute.<ExecuteActionFilterAsyncCore>d__0.MoveNext()
 --- End of stack trace from previous location where exception was thrown ---
    at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
    at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
    at System.Web.Http.Controllers.ActionFilterResult.<ExecuteAsync>d__2.MoveNext()
 --- End of stack trace from previous location where exception was thrown ---
    at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
    at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
    at System.Web.Http.Filters.AuthorizationFilterAttribute.<ExecuteAuthorizationFilterAsyncCore>d__2.MoveNext()
 --- End of stack trace from previous location where exception was thrown ---
    at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
    at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
    at System.Web.Http.Controllers.AuthenticationFilterResult.<ExecuteAsync>d__0.MoveNext()
 --- End of stack trace from previous location where exception was thrown ---
    at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
    at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
    at System.Web.Http.Controllers.ExceptionFilterResult.<ExecuteAsync>d__0.MoveNext()"}

Answer 1

The MongoDB driver itself is the culprit. When it encounters an Id property, it automatically assumes that it needs to be mapped to a corresponding _id field. In order to resolve the problem, you just need to add an attribute telling the driver that you know better than it does:

public class ChildItemViewModel
{
    [BsonElement("Id")]
    public long Id { get; set; }
    public string Description { get; set; }
    public long SequentialOrgId { get; set; }
}

Alternatively, it appears that there is also the [BsonNoId] attribute as well, which you apply at the class-level:

[BsonNoId]
public class ChildItemViewModel