在 Mongodb C# 中，分组时，如何创建包含全部内容的组列表？

Question

我正在使用 Mongo C# v 2.11.5 按用户的 CategoryId 对用户进行分组。我希望变量 list_of_groups 包含一个组列表，其中每个组都有一个组中所有文档的列表，其中每个文档都包含其所有单独的属性。我不想在任何字段上应用任何累加器函数。

UserGroup 类是：

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

我的分组查询是

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

我想创建一个“列表列表”，其中每个内部列表都包含其分组的所有文档，即这种类型的结构：

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)
)

不起作用，因为在运行时会生成以下错误：

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

谁能指出我在这里遗漏了什么？我已经转了一段时间了。我是否应该不使用 Group 函数，也许其他一些函数更合适，因为我不想应用任何累加器函数，并且因为我希望每个组中的每个文档都有其所有属性可用？

堆栈跟踪：

   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)

Answer 1

您在这里面临的问题是 .Group() 方法中的所有内容都应转换为相应的 MongoDB / 聚合框架运算符，显然没有 .ToList() 对应物。因此，如果您想返回文档列表并且必须直接使用聚合框架的语法并利用 $$ROOT 变量，那么 LINQ 没有前进的道路：

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();