如何在MongoDB C#聚合管道中使用Addfields

问题描述

Mongo DB的Aggregation管道具有"AddFields"阶段，该阶段使您可以将新字段投影到管道的输出文档中，而无需知道已经存在哪些字段.

Mongo DB's Aggregation pipeline has an "AddFields" stage that allows you to project new fields to the pipeline's output document without knowing what fields already existed.

似乎这未包含在Mongo DB的C#驱动程序中(使用2.7版).

It seems this has not been included in the C# driver for Mongo DB (using version 2.7).

有人知道这是否有其他选择吗?也许是项目"阶段的标志?

Does anyone know if there are any alternatives to this? Maybe a flag on the "Project" stage?

推荐答案

如此处所述，您可以使用BsonDocument自己构建聚合阶段.

As discussed here Using $addFields in MongoDB Driver for C# you can build the aggregation stage yourself with a BsonDocument.

要使用 https://docs.mongodb.com上的示例/manual/reference/operator/aggregation/addFields/

{
  $addFields: {
    totalHomework: { $sum: "$homework" } ,
    totalQuiz: { $sum: "$quiz" }
  }
}

看起来会像这样:

BsonDocument expression = new BsonDocument(new List<BsonElement>() {
    new BsonElement("totalHomeWork", new BsonDocument(new BsonElement("$sum", "$homework"))),
    new BsonElement("totalQuiz", new BsonDocument(new BsonElement("$sum", "$quiz")))
});
BsonDocument addFieldsStage = new BsonDocument(new BsonElement("$addFields", expression));
IAggregateFluent<BsonDocument> aggregate = col.Aggregate().AppendStage(addFieldsStage);

表达式是代表BsonDocument的

expression being the BsonDocument representing

{
  totalHomework: { $sum: "$homework" } ,
  totalQuiz: { $sum: "$quiz" }
}

您可以像往常一样将其他阶段附加到IAggregateFluent对象上

You can append additional stages onto the IAggregateFluent Object as normal

IAggregateFluent<BsonDocument> aggregate = col.Aggregate()
    .Match(filterDefintion)
    .AppendStage(addFieldsStage)
    .Project(projectionDefintion);

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