Mongo DB Merge to return only single field-CodePudding

Frinds I have the following Mongo DB query to select a random record and return the updated record using merge

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

the return of above query is

[
  {
    "_id": 3,
    "lastAccessed": ISODate("2022-02-16T13:20:38.968Z"),
    "tableName": "myTable"
  }
]

However I want to tweak above query and return only tableName instead , please let me know how it can be done ?

CodePudding user response：

You can use $project in your aggregation pipeline to show/hide the fields you want.

db.collection.aggregate([
  {
    $match: {
      "lastAccessed": {
        $exists: false
      }
    }
  },
  {
    "$sample": {
      "size": 1
    }
  },
  {
    $set: {
      tableName: "myTable",
      lastAccessed: "$$NOW"
    }
  },
  {
    "$merge": {
      "into": "collection",
      "on": "_id",
      "whenMatched": "replace"
    }
  },
  {
     $project: {
      "tableName": 1,       
      " _id": 0      
    }   
  }
])

will give [{"tableName": "myTable"}]