Mongodb- Delete some of the array elements within embedded document

Question

There are 2 kinds of documents. Type 1. Documents contain the either MCN-ONE, MCN-TWO, MCN-THREE(or all 3) along with other values 2. Another type of documents do not contain any among these values. First, I would like to get the documents having those array elements(either 1 or 2 or all 3). Then I want to keep MCN-ONE,MCN-TWO,MCN-THREE and delete all others (CCC-ALARM..etc) in bulk. Could you help to write the query? The below mentioned document falls in type 1.

{
"_id" : ObjectId("5d721f5296eaaafd1df263e8"),
"assetId" : "ALL",
"createdTime" : ISODate("2019-09-06T08:56:50.065Z"),
"default" : false,
"lastUpdatedTime" : ISODate("2019-09-06T09:11:35.463Z"),
"preferences" : {
    "MCN-TWO" : [ 
        "TEST"
    ],
    "MCN-ONE" : [ 
        "TEST", 
        "TEST", 
        "TEST"
    ],
    "MCN-THREE" : [ 
        "TEST"
    ],
    "CCC-ALARM" : [ 
        "TEST"
    ],
    "SSD-ALARM" : [ 
        "TEST"
    ],
    "TFT-ALARM" : [ 
        "TEST", 
        "TEST"
    ],
    "REC-WARN" : []
}
}

Wernfried Domscheit · Accepted Answer · 2020-03-18 10:42:20Z

1

The generic approach would be to transform preferences subdocument with $objectToArray, then filter desired elements with $filter, $map or $reduce and transform back with $arrayToObject. However, your requirement is "get elements MCN-ONE,MCN-TWO,MCN-THREE". The simple way is to update element preferences and replace just with conten of MCN-ONE,MCN-TWO,MCN-THREE. It can be done by this aggregation:

In order to filter documents, set the $match stage:

db.collection.aggregate(
   [
      {
         $match: {
            $expr: {
               $or: [
                  { $ne: ["$preferences.MCN-ONE", null] },
                  { $ne: ["$preferences.MCN-TWO", null] },
                  { $ne: ["$preferences.MCN-THREE", null] }
               ]
            }
         }
      },
      {
         $set: {
            preferences: {
               $mergeObjects: [
                  { "MCN-ONE": "$preferences.MCN-ONE" },
                  { "MCN-TWO": "$preferences.MCN-TWO" },
                  { "MCN-THREE": "$preferences.MCN-THREE" }
               ]
            }
         }
      }
   ]
)

edited Mar 18, 2020 at 10:42

answered Mar 17, 2020 at 14:27

Wernfried Domscheit

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9 Comments

San Over a year ago

Can you explain this query.?

San Over a year ago

Thanks for the explanation. Looks I have not explained properly my use case.Updated the question.

Wernfried Domscheit Over a year ago

According to what you have asked, the solution is still the same.

San Over a year ago

But i suspect, the above query would update the type 2 documents as well which i don't want to touch. Am i wrong here?

Wernfried Domscheit Over a year ago

See my answer. $set was added in MongoDB 4.2, use $addFields instead.

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