Mongo C# Driver 2.0 Aggregate Group exception

Question

I am attempting to select the latest record in a collection for each group according to a multi-field key, using the fluent Aggregate interface:

        var matches = await Collection.Aggregate()
            .Match(x => x.EffectiveDate >= minEffectiveDate)
            .SortByDescending(x => x.LastUpdate)
            .Group(key => new { key.EffectiveDate, key.ProductOid, key.InstrumentParentOid, key.ComponentOid, key.EventSummary }, g => g.First())
            .ToListAsync();

However, I get the following exception:

System.InvalidCastException occurred
  HResult=-2147467262
  Message=Unable to cast object of type 'MongoDB.Driver.Linq.Expressions.SerializationExpression' to type 'System.Linq.Expressions.MethodCallExpression'.
  Source=MongoDB.Driver
  StackTrace:
   at MongoDB.Driver.Linq.Processors.GroupSerializationInfoBinder.GetBodyFromSelector(MethodCallExpression node)
   at MongoDB.Driver.Linq.Processors.GroupSerializationInfoBinder.GetAggregationArgument(MethodCallExpression node)
   at MongoDB.Driver.Linq.Processors.GroupSerializationInfoBinder.VisitMethodCall(MethodCallExpression node)
   at MongoDB.Driver.Linq.Translators.AggregateProjectionTranslator.BindSerializationInfo(SerializationInfoBinder binder, LambdaExpression node, IBsonSerializer parameterSerializer)
   at MongoDB.Driver.Linq.Translators.AggregateProjectionTranslator.TranslateGroup[TKey,TDocument,TResult](Expression`1 idProjector, Expression`1 groupProjector, IBsonSerializer`1 parameterSerializer, IBsonSerializerRegistry serializerRegistry)
   at MongoDB.Driver.IAggregateFluentExtensions.GroupExpressionProjection`3.Render(IBsonSerializer`1 documentSerializer, IBsonSerializerRegistry serializerRegistry)
   at MongoDB.Driver.AggregateFluent`2.<>c__DisplayClass1`1.<Group>b__0(IBsonSerializer`1 s, IBsonSerializerRegistry sr)
   at MongoDB.Driver.DelegatedPipelineStageDefinition`2.Render(IBsonSerializer`1 inputSerializer, IBsonSerializerRegistry serializerRegistry)
   at MongoDB.Driver.PipelineStageDefinition`2.MongoDB.Driver.IPipelineStageDefinition.Render(IBsonSerializer inputSerializer, IBsonSerializerRegistry serializerRegistry)
   at MongoDB.Driver.PipelineStagePipelineDefinition`2.Render(IBsonSerializer`1 inputSerializer, IBsonSerializerRegistry serializerRegistry)
   at MongoDB.Driver.MongoCollectionImpl`1.<AggregateAsync>d__7`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.IAsyncCursorSourceExtensions.<ToListAsync>d__14`1.MoveNext()

Ok. So it doesn't like the IEnumerable extension method for some reason. So I try extracting the LINQ extension method:

        var matches = await Collection.Aggregate()
            .Match(x => x.EffectiveDate >= minEffectiveDate)
            .SortByDescending(x => x.LastUpdate)
            .Group(key => new { key.EffectiveDate, key.ProductOid, key.InstrumentParentOid, key.ComponentOid, key.EventSummary }, g => g)
            .ToListAsync();
        return matches.Select(x => x.First());

But:

System.InvalidCastException occurred
  HResult=-2147467262
  Message=Unable to cast object of type 'MongoDB.Bson.Serialization.Serializers.ArraySerializer`1[SPMO.Providers.Audit.Messages.ProductAdjustmentAuditDataDb]' to type 'MongoDB.Bson.Serialization.IBsonSerializer`1[System.Linq.IGrouping`2[<>f__AnonymousType0`5[System.Nullable`1[System.DateTime],System.Nullable`1[System.Int32],System.Nullable`1[System.Int32],System.Nullable`1[System.Int32],System.String],SPMO.Providers.Audit.Messages.ProductAdjustmentAuditDataDb]]'.
  Source=MongoDB.Driver
  StackTrace:
       at MongoDB.Driver.Linq.Translators.AggregateProjectionTranslator.TranslateGroup[TKey,TDocument,TResult](Expression`1 idProjector, Expression`1 groupProjector, IBsonSerializer`1 parameterSerializer, IBsonSerializerRegistry serializerRegistry)
       at MongoDB.Driver.IAggregateFluentExtensions.GroupExpressionProjection`3.Render(IBsonSerializer`1 documentSerializer, IBsonSerializerRegistry serializerRegistry)
       at MongoDB.Driver.AggregateFluent`2.<>c__DisplayClass1`1.<Group>b__0(IBsonSerializer`1 s, IBsonSerializerRegistry sr)
       at MongoDB.Driver.DelegatedPipelineStageDefinition`2.Render(IBsonSerializer`1 inputSerializer, IBsonSerializerRegistry serializerRegistry)
       at MongoDB.Driver.PipelineStageDefinition`2.MongoDB.Driver.IPipelineStageDefinition.Render(IBsonSerializer inputSerializer, IBsonSerializerRegistry serializerRegistry)
       at MongoDB.Driver.PipelineStagePipelineDefinition`2.Render(IBsonSerializer`1 inputSerializer, IBsonSerializerRegistry serializerRegistry)
       at MongoDB.Driver.MongoCollectionImpl`1.<AggregateAsync>d__7`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.IAsyncCursorSourceExtensions.<ToListAsync>d__14`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.Runtime.CompilerServices.TaskAwaiter`1.GetResult()

My plan B is just to forget about Aggregate, do a simple Find() and then do a GroupBy() in vanilla LINQ, but I would like to get this working on the DB as it will be more efficient.

Answer 1

I ran across the same issue. My workaround is different: rather than using LINQ GroupBy() client side, I make two server calls.

1. Get the MongoDB ObjectIds of the documents I wish to retrieve, i.e., the documents I wish the Group() would simply emit for me as POCOs.
2. Do a Find() with Contains() against the collection of ObjectIds from step 1.

Code demonstrating this

public static class WidgetWorker
{
    public static void Main()
    {
        //setup
        var widgetCollection = new MongoClient("mongodb://localhost:27017")
            .GetDatabase("WidgetDatabase")
            .GetCollection<Widget>("Widget");
        widgetCollection.DeleteManyAsync(x => true).Wait();   //remove all existing rows

        //create widgets and add to DB;  2 SKUs, each with multiple revisions
        var widgetA1 = new Widget() { SKU = "aaaa", Revision = 1M, Cost = 10 };
        var widgetA2 = new Widget() { SKU = "aaaa", Revision = 2.1M, Cost = 20 };
        var widgetA3 = new Widget() { SKU = "aaaa", Revision = 2.2M, Cost = 30 };
        var widgetB1 = new Widget() { SKU = "bbbb", Revision = 1M, Cost = 40 };
        var widgetB2 = new Widget() { SKU = "bbbb", Revision = 1.1M, Cost = 50 };
        widgetCollection.InsertManyAsync(new[] { widgetA1, widgetA2, widgetA3, widgetB1, widgetB2 }).Wait();

        //get the ObjectId of the most Recent revision of each SKU
        var r = widgetCollection
            .Aggregate()
            .SortByDescending(x => x.Revision)
            .Group(x => x.SKU, g => new { Id = g.First().Id })
            .ToListAsync()
            .Result;

        //get the Widget objects for the list of ids just collected
        var ids = r.Select(x => x.Id).ToArray();
        var widgets = widgetCollection
            .Find(x => ids.Contains(x.Id))
            .ToListAsync()
            .Result;

        //check results
        Debug.Assert(widgets.Count() == 2);
        Debug.Assert(widgets.Single(x => x.SKU == "aaaa").Revision == 2.2M);
        Debug.Assert(widgets.Single(x => x.SKU == "bbbb").Revision == 1.1M);
    }
}

public class Widget
{
    [BsonId]
    public ObjectId Id { get; set; }
    public string SKU { get; set; }
    public decimal Revision { get; set; }
    public decimal Cost { get; set; }
}