MongoDB：在 $look 阶段后将新字段添加到现有子文档或将查找响应合并到主文档

Question

I want new field "isActive" inside modifierStatus sub document which is coming from modifieritems collection.

modifieritems collection :

{
    "_id" : ObjectId("5e6a5a0e6d40624b12453a67"),
    "modifierName" : "xxx",
    "isActive" : 1
}
{
    "_id" : ObjectId("5e6a5a0e6d40624b12453a6a"),
    "modifierName" : "yyy",
    "isActive" : 0
}

favoritedrinks collection :

{
  "alcoholName" : "whiskey",
  "modifierList" : [{
       "_id" : ObjectId("5e6a5a0e6d40624b12453a61"), 
       "modifierId" : ObjectId("5e6a5a0e6d40624b12453a67"),
       "modifierName" : "xxx",
     }
     {
       "_id" : ObjectId("5e6a5a0e6d40624b12453a66"),
       "modifierId" : ObjectId("5e6a5a0e6d40624b12453a6a"),
       "modifierName" : "yyy",
     }]
 }

my query is :

db.getCollection('favoritedrinks').aggregate([
  { "$sort": { "alcoholName": 1 } },
  {"$lookup": {
      "from": "modifieritems",
      localField: 'modifierList.modifierId', 
      foreignField: '_id', 
      as: 'modifier'
  }},
  {
     $project:{
        "alcoholName" : "$alcoholName",
        "modifierStatus":"$modifier", 
     }
   },
 ]);

But my expected result :

{
     "alcoholName" : "Whiskey",
      "modifierStatus" : [
        {
            "_id" : ObjectId("5e6a5a0e6d40624b12453a61"),
            "modifierId" : ObjectId("5e6a5a0e6d40624b12453a67"), 
            "modifierName" : "xxx",
            "isActive" : 1,
        },
        {
            "_id" : ObjectId("5e6a5a0e6d40624b12453a66"),
            "modifierId" : ObjectId("5e6a5a0e6d40624b12453a6a"),
            "modifierName" : "yyy",
            "isActive" : 0,
        }
    ]
}

anyone please help me?

Answer 1

Try this query :

Update with new requirement :

db.favoritedrinks.aggregate([
  {
    "$sort": {
      "alcoholName": 1
    }
  },
  {
    "$lookup": {
      "from": "modifieritems",
      localField: "modifierList.modifierId",
      foreignField: "_id",
      as: "modifierStatus"
    }
  },
  {
    $addFields: {
      modifierStatus: {
        $map: {
          input: "$modifierList",
          as: "m",
          in: {
            $mergeObjects: [
              {
                $arrayElemAt: [ /** As filter would only get one object (cause you'll have only one matching doc in modifieritems coll for each "modifierList.modifierId", So getting first element out of array, else you need to take this array into an object & merge that field to particular object of 'modifierList') */ 
                  {
                    $filter: {
                      input: "$modifierStatus",
                      cond: {
                        $eq: [
                          "$$this._id",
                          "$$m.modifierId"
                        ]
                      }
                    }
                  },
                  0
                ]
              },
              "$$m"
            ]
          }
        }
      }
    }
  },
  {
    $project: {
      modifierStatus: 1,
      alcoholName: 1,
      _id: 0
    }
  }
])

Test : MongoDB-Playground

Old :

db.favoritedrinks.aggregate([
    {
        "$sort": {
            "alcoholName": 1
        }
    },
    {
        $lookup: {
            from: "modifieritems",
            let: {
                id: "$modifierList.modifierId"
            },
            pipeline: [
                {
                    $match: { $expr: { $in: ["$_id", "$$id"] } }
                },
                /** Adding a new field modifierId(taken from _id field of modifieritems doc) 
                 * to each matched document from modifieritems coll */
                {
                    $addFields: {
                        modifierId: "$_id"
                    }
                }
            ],
            as: "modifierStatus"
        }
    },
    /** By mentioning 0 to particular fields to remove them & retain rest all other fields */
    {
        $project: {
            modifierList: 0,
            _id: 0
        }
    }
])

Test : MongoDB-Playground

Answer 2

When you want $project to include a field's current value while keeping the same field name, you need only specify :1 . When you use "$field" you are explicitly setting the value, which will overwrite any existing value.

Try making your projection:

{
     $project:{
        "alcoholName" : 1,
        "modifier.isActive": 1,
        "modifier.modifierName": 1 
     }
}