How can I project an object only if document has a specific field in MongoDB?

Question

I have the following project:

{
    '$project': {
        '_id': 1,
        'applicationIds': {
            'type': 'linux',
            'refId': '$applicationIds'
        },
        'configurationIds': {
            'type': 'linux',
            'refId': '$configurationIds'
        }
    }
}

As long as there is at least one record in $configurationIds I will get something like this:

{
    'type': 'linux',
    'refId': ObjectId('...')
}

And same with $applicationIds..

The issue arises when there are no records in $configurationIds or $applicationIds then I get this:

{
    'type': 'linux'
}

I don't want this, if there are no $applicationIds, I just want the object to be empty.

Btw I do an $unwind on $applicationIds (with preserveNullAndEmptyArrays) just before this stage so there can only be either one or no applicationIds in each document. Same goes for applicationIds.

so there can only be either one or no applicationIds in each document. is it same for he other field — cmgchess, Apr 19 '23 at 13:44
so before projecting is it like `{ "_id": 1, "applicationIds": ObjectId(blabla)}`. also if no object id there is no field — cmgchess, Apr 19 '23 at 13:48
Yes, that's all correct., and that's the case for both applicationIds and configurationIds. — TheStranger, Apr 19 '23 at 13:52
again if i understand corect u need sommething like `{ "_id": 1, "applicationIds": { "refId": ObjectId("adfghjk"), "type": "linux" }, "configurationIds": {} }` if configurationIds field not exists? — cmgchess, Apr 19 '23 at 14:12
Yes, or you don’t even need the configuratuonIds if it doesn’t exist.. just project it if it exists exists, that would be fine too if it’s easier.. — TheStranger, Apr 19 '23 at 14:27
can you check the below one i just assumed you need an empty object if doesn't exist — cmgchess, Apr 19 '23 at 14:28
i gave test cases for both the cases you mentioned. you can have a look — cmgchess, Apr 19 '23 at 14:36

cmgchess · Accepted Answer · 2023-04-19T14:35:44.053

you can write a condition to check if the field is missing or not. This post shows a way to check that and if doesn't exists project an empty object

db.collection.aggregate([
  {
    $project: {
      _id: 1,
      applicationIds: {
        $cond: {
          if: { $ne: [ { $type: "$applicationIds" }, "missing" ] },
          then: { type: "linux", refId: "$applicationIds" },
          else: {}
        }
      },
      configurationIds: {
        $cond: {
          if: { $ne: [ { $type: "$configurationIds" }, "missing" ] },
          then: { type: "linux", refId: "$configurationIds" },
          else: {}
        }
      }
    }
  }
])

playground

if you don't want the non existing field after projecting as well instead of the empty object you can use $$REMOVE. This post has some example usage of it

db.collection.aggregate([
  {
    $project: {
      _id: 1,
      applicationIds: {
        $cond: {
          if: { $ne: [ { $type: "$applicationIds" }, "missing" ] },
          then: { type: "linux", refId: "$applicationIds" },
          else: "$$REMOVE"
        }
      },
      configurationIds: {
        $cond: {
          if: { $ne: [ { $type: "$configurationIds" }, "missing" ] },
          then: { type: "linux", refId: "$configurationIds" },
          else: "$$REMOVE"
        }
      }
    }
  }
])

playground

score 0 · Answer 2 · answered Apr 20 '23 at 12:38

0

db.myCollection.aggregate([
   {
      $project: {
         fieldToProject: {
            $cond: {
               if: { $exists: ["$specificField", true] },
               then: "$specificField",
               else: null
            }
         }
      }
   }
])

answered Apr 20 '23 at 12:38

Rahul sharma

9

did you test this. says exists doesnt work here – cmgchess Apr 20 '23 at 15:17
As it’s currently written, your answer is unclear. Please [edit] to add additional details that will help others understand how this addresses the question asked. You can find more information on how to write good answers [in the help center](/help/how-to-answer). – Community Apr 24 '23 at 20:20

How can I project an object only if document has a specific field in MongoDB?

2 Answers2