MongoDb: Select a random record with a specifi condition and update it-CodePudding

Friends

I have a collection with around 20 records of lastAccessed as null and now I want to select a random lastAccessed and update it with $$Now for which I have the below code

    db.myCollection.aggregate([
      { $match: { lastAccessed: { $exists: False}  }},
      { $sample:{ size: 1 }}
    ]);
    db.myCollection.updateOne(
      { _id: 1 },
      {
        $set: {
            tableName: 'myTable',
            lastAccessed: '$$NOW'
        }
      })

My question is how do I know the id of the randomly selected record to update it correctly ?

CodePudding user response：

For your scenario, you can use $merge at the end of the aggregation pipeline to perform update by id behaviour.

db.collection.aggregate([
  {
    $match: {
      "lastAccessed": {
        $exists: false
      }
    }
  },
  {
    "$sample": {
      "size": 1
    }
  },
  {
    $set: {
      tableName: "myTable",
      lastAccessed: "$$NOW"
    }
  },
  {
    "$merge": {
      "into": "collection",
      "on": "_id",
      "whenMatched": "replace"
    }
  }
])

Here is the Mongo playground for your reference.

CodePudding user response：

If we don't want to use aggregate,

To get Updated Document, can use findOneAndUpdate()

db.collection.findOneAndUpdate(
    { lastAccessed: { $exists: false } },
    { $set: { lastAccessed: "$$NOW" } },
    { returnOriginal: false }
)

Use { returnOriginal: true | false } option for Mongodb 3.6 earlier version,

For MongoDb 3.6 and after 3.6 Version, can use { returnDocument: "after", "before" },

{ new: true | false } option for mongoose