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I am using Mongo C# v 2.11.5 to group users by their CategoryId. I would like the variable list_of_groups to contain a list of groups, where each group has a list of all the documents in the group, where each document contains all its individual properties. I don't want to apply any accumulator functions on any fields.

The UserGroup class is:

public class UserGroup { public string gid {get; set;} public List<Users> users { get; set; } }

My grouping query is

 var list_of_groups = UserCollection .Aggregate() .Group(usr => usr.CategoryId, g => new UserGroup { gid = g.Key, users = g.ToList() }) .ToList<UserGroup>();

I would like to create a "list of lists", where each inner list contains all the documents of its grouping i.e. this type of structure:

List<List<UserGroup>>( - List1<UserGroup> (list of complete documents belonging to CategoryId 1) - List2<UserGroup> (list of complete documents belonging to CategoryId 2) - List3<UserGroup> (list of complete documents belonging to CategoryId 3) )

The doesn't work as the following error is generated at runtime:

System.NotSupportedException HResult=0x80131515 Message=Specified method is not supported. Source=MongoDB.Driver

Can anyone point out what I'm missing here? I've been going in circles for a while now. Should I not being using the Group function, and maybe some other function is more appropriate since I don't want to apply any accumulator functions, and because I want each document in each group to have all its properties available?

Stack Trace:

 at MongoDB.Driver.Linq.Processors.AccumulatorBinder.GetAccumulatorArgument(Expression node) at MongoDB.Driver.Linq.Processors.AccumulatorBinder.TryGetAccumulatorTypeAndArgument(PipelineExpression node, AccumulatorType& accumulatorType, Expression& argument) at MongoDB.Driver.Linq.Processors.AccumulatorBinder.VisitPipeline(PipelineExpression node) at MongoDB.Driver.Linq.Expressions.PipelineExpression.Accept(ExtensionExpressionVisitor visitor) at MongoDB.Driver.Linq.Expressions.ExtensionExpression.Accept(ExpressionVisitor visitor) at System.Linq.Expressions.ExpressionVisitor.Visit(Expression node) at MongoDB.Driver.Linq.Processors.AccumulatorBinder.Bind(Expression node, IBindingContext bindingContext) at MongoDB.Driver.Linq.Processors.EmbeddedPipeline.EmbeddedPipelineBinder.Bind(Expression node, IBindingContext parent) at MongoDB.Driver.Linq.Processors.SerializationBinder.BindEmbeddedPipeline(MethodCallExpression node) at MongoDB.Driver.Linq.Processors.SerializationBinder.VisitMethodCall(MethodCallExpression node) at System.Linq.Expressions.MethodCallExpression.Accept(ExpressionVisitor visitor) at System.Linq.Expressions.ExpressionVisitor.Visit(Expression node) at MongoDB.Driver.Linq.Processors.SerializationBinder.Visit(Expression node) at System.Linq.Expressions.ExpressionVisitor.VisitMemberAssignment(MemberAssignment node) at System.Linq.Expressions.ExpressionVisitor.VisitMemberBinding(MemberBinding node) at System.Linq.Expressions.ExpressionVisitor.Visit[T](ReadOnlyCollection`1 nodes, Func`2 elementVisitor) at System.Linq.Expressions.ExpressionVisitor.VisitMemberInit(MemberInitExpression node) at System.Linq.Expressions.MemberInitExpression.Accept(ExpressionVisitor visitor) at System.Linq.Expressions.ExpressionVisitor.Visit(Expression node) at MongoDB.Driver.Linq.Processors.SerializationBinder.Visit(Expression node) at MongoDB.Driver.Linq.Processors.SerializationBinder.Bind(Expression node, IBindingContext context, Boolean isClientSideProjection) at MongoDB.Driver.Linq.Processors.PipelineBindingContext.Bind(Expression node, Boolean isClientSideProjection) at MongoDB.Driver.Linq.Processors.PipelineBindingContext.Bind(Expression node) at MongoDB.Driver.Linq.Translators.AggregateGroupTranslator.BindGroup[TKey,TDocument,TResult](PipelineBindingContext bindingContext, Expression`1 groupProjector, IBsonSerializer`1 parameterSerializer, Expression keySelector) at MongoDB.Driver.Linq.Translators.AggregateGroupTranslator.Translate[TKey,TDocument,TResult](Expression`1 idProjector, Expression`1 groupProjector, IBsonSerializer`1 parameterSerializer, IBsonSerializerRegistry serializerRegistry, ExpressionTranslationOptions translationOptions) at MongoDB.Driver.GroupExpressionProjection`3.Render(IBsonSerializer`1 documentSerializer, IBsonSerializerRegistry serializerRegistry) at MongoDB.Driver.PipelineStageDefinitionBuilder.<>c__DisplayClass19_0`2.<Group>b__0(IBsonSerializer`1 s, IBsonSerializerRegistry sr) at MongoDB.Driver.DelegatedPipelineStageDefinition`2.Render(IBsonSerializer`1 inputSerializer, IBsonSerializerRegistry serializerRegistry) at MongoDB.Driver.AppendedStagePipelineDefinition`3.Render(IBsonSerializer`1 inputSerializer, IBsonSerializerRegistry serializerRegistry) at MongoDB.Driver.MongoCollectionImpl`1.Aggregate[TResult](IClientSessionHandle session, PipelineDefinition`2 pipeline, AggregateOptions options, CancellationToken cancellationToken) at MongoDB.Driver.MongoCollectionImpl`1.<>c__DisplayClass19_0`1.<Aggregate>b__0(IClientSessionHandle session) at MongoDB.Driver.MongoCollectionImpl`1.UsingImplicitSession[TResult](Func`2 func, CancellationToken cancellationToken) at MongoDB.Driver.MongoCollectionImpl`1.Aggregate[TResult](PipelineDefinition`2 pipeline, AggregateOptions options, CancellationToken cancellationToken) at MongoDB.Driver.CollectionAggregateFluent`2.ToCursor(CancellationToken cancellationToken)
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    What do you expect from the .Aggregate()? Here is the doc on this method. learn.microsoft.com/en-us/dotnet/api/…. Commented Jan 6, 2021 at 13:51
  • Does this answer your question? Using Linq to group a list of objects into a new grouped list of list of objects Commented Jan 6, 2021 at 13:51
  • @DragandDrop Not sure we're considering the same Aggregate() method- I'm using the IMongoCollectionExtensions.Aggregate which returns an IAggregateFluent<TDocument> from mongodb.github.io/mongo-csharp-driver/2.8/apidocs/html/… Commented Jan 6, 2021 at 14:52
  • Ma bad. Never used that one. I missed the tag aggregation-framework, that was a good hint of it beeing a framework specific method. For my defence I often see code with naked Aggregate not related to that framework, as it's the first method in the Crtl space suggestion. It's sometimes use as a place holder or go-get-it method. Commented Jan 7, 2021 at 8:36

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The problem you're facing here is that everything that's within .Group() method should get translated into corresponding MongoDB / Aggregation Framework operator and obviously there's no .ToList() counterpart. Therefore, there's no path forward with LINQ if you want to return a list of documents and you have to use Aggregation Framework's syntax directly and take advantage of $$ROOT variable:

public class UserGroup { [BsonElement("_id")] public string gid { get; set; } public List<User> users { get; set; } } var groupDef = new BsonDocument() { { "_id", "$CategoryId" }, { "users", new BsonDocument(){ { "$push", "$$ROOT" } } } }; var res = collection.Aggregate().Group<UserGroup>(groupDef).ToList();
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2 Comments

How can this example be adapted, if only the groups containing more than n users (for example 2) should be returned? Is it possible to write the query without using the BsonDocument (linq or fluentaggregate style) using the new linq3 provider from the mongodb c# driver 2.14?
@RickyTad please open another question and describe your case

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