C＃中的MongoDB查找和过滤

Question

在C＃中执行查找聚合后，我在过滤mongodb中的文档时遇到了这个问题。我正在使用多个查询，因此需要从上一个子集合中过滤掉文档。以下是收藏列表：

集合1： ID，ParentId，Desc，索引

集合2 ID，ParentId，Desc，索引，Coll2Desc1，Coll2Desc2

集合3 ID，ParentId，Desc，索引，Coll3Desc1，Coll3Desc2

集合4 ID，ParentId，Desc，索引，Coll4Desc1，Coll4Desc2

现在，我需要过滤并仅获取Coll4Desc1不为null或为空的那些记录。我尝试在投影代码后添加$ match，但不起作用。如果有人可以帮助我过滤掉所需的文档，那将非常好。

以下是我编写的查找聚合代码：

var collectionList = list.OrderBy(x => x.CollectionOrder).Select(x => x.CollectionName).Distinct().ToList();


            // Load data into deviations 
            var listOfDocuments = new List<BsonDocument>();

            var firstCollection = dbContext.GetCollection(collectionList[0]);
            var options = new AggregateOptions()
            {
                AllowDiskUse = false
            };
            var previousCollectionName = "";
            var sortProject = new BsonDocument();
            var groupBy = new List<BsonDocument>();
            for (int i = 0; i < collectionList.Count - 1; i++)
            {
                var collectionName = collectionList[i];
                IMongoCollection<BsonDocument> collection = dbContext.GetCollection(collectionName);
                if (i == 0)
                {
                    firstCollection = collection;
                    var firstarray = new BsonDocument("$project", new BsonDocument()
                        .Add("_id", 0)
                        .Add(collectionName, "$$ROOT"));

                    listOfDocuments.Add(firstarray);
                }
                else
                {
                    // join all the collections

                    var remainingArray = new BsonDocument("$lookup", new BsonDocument()
                            .Add("localField", previousCollectionName + "." + "Id")
                            .Add("from", collectionName)
                            .Add("foreignField", "ParentId")
                            .Add("as", collectionName));
                    listOfDocuments.Add(remainingArray);

                    remainingArray = new BsonDocument("$unwind", new BsonDocument()
                            .Add("path", "$" + collectionName)
                            .Add("preserveNullAndEmptyArrays", new BsonBoolean(true)));

                    listOfDocuments.Add(remainingArray);


                    sortProject.Add(previousCollectionName + "." + "Id", 1);

                }
                previousCollectionName = collectionName;
            }
            // Project the columns 
            var columnList = list.OrderBy(x => x.ColumnOrder).Where(x => x.IsVisible == true).ToList();
            var docProjection = new BsonDocument();

            for (int i = 0; i < columnList.Count; i++)
            {
                docProjection.Add(columnList[i].ColumnName, "$" + columnList[i].CollectionName + "." + columnList[i].FieldName);

            }

            listOfDocuments.Add(new BsonDocument("$project", docProjection));
            listOfDocuments.Add(new BsonDocument("$sort", sortProject));  
             var match2 = new BsonDocument
                {
                    {
                        "$match",
                        new BsonDocument
                            {
                                {"Collection4.Coll4Desc1", new BsonDocument
                                    {
                                        {
                                            "$ne", ""
                                        }
                                    }
                                }
                            }
                    }
                };
            listOfDocuments.Add(match2);
        PipelineDefinition<BsonDocument, BsonDocument> pipeline = listOfDocuments;

            var listOfDocs = new List<BsonDocument>();
            firstCollection.AggregateAsync(pipeline, options))
            using (var cursor = firstCollection.Aggregate(pipeline, options))
            {

                while (cursor.MoveNext())
                {
                    var batch = cursor.Current;
                    foreach (BsonDocument document in batch)
                    {
                        listOfDocs.Add(document);
                    }
                }
            }