猫鼬：在聚合中过滤 $lookup

Question

我在猫鼬中有以下聚合代码，它运行良好，但我正在寻找过滤结果：

Mov.aggregate(
       [
              { $sort: { _id: 1, car_id: 1, area: 1 } },
         {
           $group:
             {
               _id: "$car_id",
               lastLocation: { $last: "$area" }
             }
         },
                     {
            $lookup: {
                from: Car.collection.name,
                "localField": "_id",
                "foreignField": "_id",
                as: 'cars'
            }
        },    
      ]
         )
        .then(movs => {
            res.send(movs);
        }).catch(err => {
            res.status(500).send({
                message: err.message
            });
 });

如何过滤$lookup 中的Car.collection.name，我只想获得带有{"active":true} 的汽车？

Answer 1

您可以使用不同的查找策略。以下是更新后的查询：

Mov.aggregate([{
    $sort: {
      "_id": 1,
      "car_id": 1,
      "area": 1
    }
  },
  {
    $group: {
      _id: "$car_id",
      lastLocation: {
        $last: "$area"
      }
    }
  },
  {
    $lookup: {
      from: Car.collection.name,
      let: {
        "_id": "$_id"
      },
      pipeline: [{
        $match: {
          $expr: {
            $eq: ["$_id", "$$_id"]
          },
          active: true
        }
      }],
      "as": "cars"
    }
  }
]).then(movs => {
  res.send(movs);
}).catch(err => {
  res.status(500).send({
    message: err.message
  });
});

您可以查看以下链接以获取有关此查找策略https://docs.mongodb.com/manual/reference/operator/aggregation/lookup/的更多信息