Question 115287 in Mongodb Question posted in
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Mongodb – How can I project an object only if a specific field exists?

I have the following documents in my MongoDB Aggregation stage:

_id: ObjectId('641316fd596514e63a2121ca'),
teamId: 5,
playerId: ObjectId('64131139596514e63a2121c1')

_id: ObjectId('641316fd596514e63a2121ca'),
teamId: 5,
playerId: ObjectId('64143f7702ede13fea5ff3be')

_id: ObjectId('64143f458b6a6cf54652d3a5'),
teamId: 17,
playerId: ObjectId('64143fa75a97b9a63a260e16')

_id: ObjectId('64143f4f9ba5f2b3cf3030e6'),
teamId: 3

I want to create an Object field called players, and this object should contain the playerId and a gender: 'male', but only if the document has a playerId property already. If not, it should just not do anything. So the result should look like this:

_id: ObjectId('641316fd596514e63a2121ca'),
teamId: 5,
players: {
    gender: 'male',
    playerId: ObjectId('64131139596514e63a2121c1')
}

_id: ObjectId('641316fd596514e63a2121ca'),
teamId: 5,
players: {
    gender: 'male',
    playerId: ObjectId('64143f7702ede13fea5ff3be')
}

_id: ObjectId('64143f458b6a6cf54652d3a5'),
teamId: 17,
players: {
    gender: 'male',
    playerId: ObjectId('64143fa75a97b9a63a260e16')
}

_id: ObjectId('64143f4f9ba5f2b3cf3030e6'),
teamId: 3

I’ve tried to do this:

db.collection.aggregate([
  {
    $set: {
      players: {
        $cond: {
          if: {
            $ne: [
              "$players",
              undefined
            ]
          },
          then: {
            $mergeObjects: [
              "$players",
              {
                "gender": "male"
              }
            ]
          },
          else: "$$REMOVE"
        }
      }
    }
  }
])

But this adds an object even if there is no playerId property in the document.

Tags:

2

Answers


  1. 0

    Works with $cond operator.

    1. If the playerId field is not undefined, then set the players field with an object.

    2. Else remove the players field with $$REMOVE.

    db.collection.aggregate([
  {
    $project: {
      _id: 1,
      teamId: 1,
      players: {
        $cond: {
          if: {
            $ne: [
              "$playerId",
              undefined
            ]
          },
          then: {
            "gender": "male",
            "playerId": "$playerId"
          },
          else: "$$REMOVE"
        }
      }
    }
  }
])

    Demo @ Mongo Playground

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  2. 0 
    db.collection.aggregate([
  {
    $set: {
      players: {
        $cond: {
          if: {
            $ne: [
              "$playerId",
              undefined
            ]
          },
          then: {
            $mergeObjects: [
              {
                "playerId": "$playerId"
              },
              {
                "gender": "male"
              }
            ],
            
          },
          else: "$$REMOVE"
        }
      }
    }
  },
  {
    $set: {
      playerId: {
        "$cond": [
          {
            "$ne": [
              "$players",
              undefined
            ]
          },
          "$$REMOVE",
          "$playerId"
        ]
      }
    }
  }
])

    Demo at Mongo Playground

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