How can i limit each ID i pass to $in operator inside the $match stage to only 4 elements-CodePudding

I have an aggregate like this :

            const files = await File.aggregate([
              {
                $match: { facilityId: { $in: facilities } }
              },
              {
                $sort: { createdAt: 1 }
              },
              {
                $project: {
                  file: 0,
                }
              }
            ])

And i would like to have each "facility" return only 4 files, i used to do something like facilities.map(id => query(id)) but id like to speed things up in production env.

Using $limit will limit the whole query, that's not what i want, i tried using $slice in the projection stage but got en error :

MongoError: Bad projection specification, cannot include fields or add computed fields during an exclusion projection

how can i achieve that in a single query ?

Schema of the collections is :

const FileStorageSchema = new Schema({
  name: { type: String, required: true },
  userId: { type: String },
  facilityId: { type: String },
  patientId: { type: String },
  type: { type: String },
  accessed: { type: Boolean, default: false, required: true },
  file: {
    type: String, //
    required: true,
    set: AES.encrypt,
    get: AES.decrypt
  },
  sent: { type: Boolean, default: false, required: true },
},
{
  timestamps: true,
  toObject: { getters: true },
  toJSON: { getters: true }
})

And i would like to returns all fields except for the file fields that contains the encrypted blob encoded as base64.

Also: i have the feeling that my approach is not correct, what i really would like to get is being able to query all facilityId at once but limited to the 4 latest file created for each facility, i though using an aggregate would help me achieve this but im starting to think it's not how its done.

CodePudding user response：

From the question the schema is not clear. So I have two answers based on two Schemas. Please use what works for you

db.collection.aggregate([
  {
    $match: {
      facilityId: {
        $in: [
          1,
          2
        ]
      }
    }
  },
  {
    $group: {
      _id: "$facilityId",
      files: {
        $push: "$file"
      }
    }
  },
  {
    $project: {
      files: {
        $slice: [
          "$files",
          0,
          4
        ],
        
      }
    }
  }
])

Test Here

db.collection.aggregate([
  {
    $match: {
      facilityId: {
        $in: [
          1,
          2
        ]
      }
    }
  },
  {
    $project: {
      facilityId: 1,
      file: {
        $slice: [
          "$file",
          4
        ]
      }
    }
  }
])

Test Here