Get index of the last matching element of an array in MongoDB

Question

Is there a way to get the index of the last matching element in MongoDB? I'm speaking of an equivalent to JavaScript's Array.prototype.lastIndexOf. I know about $indexOfArray, but I'm looking for the last index, not the first one.

My use case

I've got a collection whose schema looks like this:

{
  // ...
  images: [
    {
      identifier: String,
      imageId: ObjectId
    }
  ]
  // ...
}

I have a record whose got duplicated elements:

{
  // ...
  images: [
    {
      identifier: '0',
      imageId: ObjectId('objectId0')
    },
    {
      identifier: '1',
      imageId: ObjectId('objectId1')
    },
    {
      identifier: '2',
      imageId: ObjectId('objectId2')
    },
    {
      identifier: '0', // <-- duplicated!
      imageId: ObjectId('objectId3')
    },
    // many more elements...
    {
      identifier: '0', // last occurence
      imageId: ObjectId('objectIdN+0')
    },
    {
      identifier: '1',
      imageId: ObjectId('objectIdN+1')
    },
    {
      identifier: '2',
      imageId: ObjectId('objectIdN+2')
    }
  ]
}

I want to remove all the elements before the last occurence of images.identifier: '0'.

Is it possible at all without the usage of JavaScript?

You can use an aggregation pipeline to get all the duplicate IDS except the last one using $limit & $skip. Once you have the IDs you can delete them one using ObjectID. — Hiren
– Hiren, Commented Nov 2, 2020 at 14:19
@Hiren how exactly do I get all the number of elements up until the last duplicate, though? I will need to pass a very specific value to $limit — Amit Beckenstein
– Amit Beckenstein, Commented Nov 2, 2020 at 14:26
Here are some ideas from which you can think further: Find duplicate urls in mongodb. — prasad_
– prasad_, Commented Nov 2, 2020 at 14:29
Do you like to remove "all behind images.identifier: '0'" or "all duplicated identifier"? — Wernfried Domscheit
– Wernfried Domscheit, Commented Nov 2, 2020 at 15:02

Wernfried Domscheit · Accepted Answer · 2020-11-02 15:01:21Z

1

Try this one:

db.collection.aggregate([
   {
      $set: {
         images: {
            $reduce: {
               input: "$images",
               initialValue: [],
               in: {
                  $cond: {
                     if: { $eq: ["$$this.identifier", "0"] },
                     then: ["$$this"], // reset array to current value
                     else: { $concatArrays: ["$$value", ["$$this"]] } // append values
                  }
               }
            }
         }
      }
   }
])

answered Nov 2, 2020 at 15:01

Wernfried Domscheit

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7 Comments

Amit Beckenstein Over a year ago

Won't this result in duplicates as well?

Wernfried Domscheit Over a year ago

No it does not. It looks for images.identifier: '0' and everything what appeared before in the array is removed. That's how I understood your requirement.

Amit Beckenstein Over a year ago

It actually works! I don't see how or why? Can you maybe be more verbose? Sorry for the hassle

Amit Beckenstein Over a year ago

@Wernfriend Docmscheit nevermind, I think I understand. Cool, thanks!

Wernfried Domscheit Over a year ago

Well then $reduce is not that easy to understand in my opinion: It goes over each elements of the array. If identifier: '0' is found then array is reset to current element. Otherwise the current element is appended to the array.

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