MongoDB Add Field in nested array by other field

Question

Hello i have simple collection:

{
    _id: 1,
    books: [
         { bookId: 55, c: 5},
         { bookId: 66, c: 6},
         { bookId: 77, c: 7},
    ]
}

How i can add new field by calulate other field? here i add field “Z” to current found object in nested array by it calculate field “C”

updateOne(
{ 
  _id : 1, 
  'books.bookId' : 66 
} ,
{
  [
      {    $addFields: { "books.$.z" : { "$sum" : ["$books.$.c", 1]  } }    }
  ]
}

Expected result:

{
    _id: 1,
    books: [
         { bookId: 55, c: 5},
         { bookId: 66, c: 6, z:7},
         { bookId: 77, c: 7},
    ]
}

I think there is a short entry (possibly using new $getField ?!), I think mango is still able to combine ‘position operator $’ + (‘varible operator reference by prefix $’ or ‘combine with $getField’) how i try in my sample

Buzz Moschetti · Accepted Answer · 2022-02-25 19:41:17Z

3

Use the aggregation pipeline in the update method to leverage the operators $map, $mergeObjects and $cond to achieve the desired results:

.updateOne(
    // The query will find docs where *at least* one bookId in the
    // books array equals 66 -- but remember it does not pinpoint
    // the location in the array!  We will tackle that in the update
    // pipeline to follow...
    { _id: 1, 'books.bookId': 66 },
    [
        { $set: {
            books: {
                $map: {
                    input: '$books',
                    in: {
                        $mergeObjects: [
                            '$$this',
                            {
                                $cond: [
                                    { $eq: ['$$this.bookId', 66] }, // IF books.bookId == 66
                                    { z: { $sum: ['$$this.c', 1] } }, // THEN merge a new field 'z' with calced value
                                    null  // ELSE merge null (a noop)
                                ]
                            }
                        ]
                    }
                }
            }
        } }    
    ]
)

edited Feb 25, 2022 at 19:41

Buzz Moschetti

7,6433 gold badges27 silver badges41 bronze badges

answered Feb 25, 2022 at 16:00

chridam

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5 Comments

padavan Over a year ago

there is more simple ? with position operator $ or ‘combine with $getField

chridam Over a year ago

Yes, it's that simple as there is no need to use the $ operator when you update with the aggregation pipeline, just leverage the above aggregation operators to do the update.

padavan Over a year ago

will wait for more options, simpler options, with more modern operators such as v5.0 $getfield or similar

Buzz Moschetti Over a year ago

chridam has provided the simplest, tightest solution. There is no "object.fldX = val" operation in agg language; you have to use $mergeObjects, in this case taking the whole input object ($$this) from the map loop and merging the little object {z: ...} into it. Update pipelines are also much more powerful and as your needs change or you have variations, you don't get blocked by the limitations of the $ operator paradigm (notably, only matching on the FIRST thing encountered)

chridam Over a year ago

@BuzzMoschetti Couldn’t have said it better, cheers for the edit too!

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