使用localField 1和foreignField 1进行查找

db.a.aggregate([
  {
    $lookup: {
      from: "b",
      localField: "1",
      foreignField: "1",
      as: "docs"
    }
  },
  {
    $replaceRoot: {
      newRoot: {
        $mergeObjects: [
          "$$ROOT",
          { $first: "$docs" }
        ]
      }
    }
  },
  {
    $unset: [ "docs", "_id" ]
  },
  {
    $group: {
      _id: "$UserName",
      doc: { $first: "$$ROOT" }
    }
  },
  {
    $replaceWith: "$doc"
  }
])

mongoplayground

最后，在经历了大量的尝试和错误，并尝试使用管道之后，我可以在聚合管道中做到这一点，我使用了unionWith和group。

var pipeline1= new BsonDocument("$unionWith", 
    new BsonDocument
        {
            { "coll", "CollectionB" }, 
            { "pipeline", 
    new BsonArray
            {
                new BsonDocument("$match", 
                new BsonDocument("UniqeId", 1))
            } }
        });

var pipeline2 = new BsonDocument("$group", 
    new BsonDocument
        {
            { "_id", 0 }, 
            { "merged", 
    new BsonDocument("$push", "$$ROOT") }
        });

var pipeline3 = new BsonDocument("$replaceRoot", 
    new BsonDocument("newRoot", 
    new BsonDocument("$mergeObjects", "$merged")));

var pipeline4 = new BsonDocument("$project", 
    new BsonDocument("_id", 0));

BsonDocument[] pipeline = new BsonDocument[] { pipeline1, pipeline2, pipeline3, pipeline4 };

var dbResponse = await collection.Aggregate<BsonDocument>(pipeline).ToListAsync();