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omidh
omidh

Reputation: 2822

MongoDB: Remove certain number of elements from start of an array

Is there a way to remove certain amounts of elements from start of an array in MongoDB?

Suppose I don't know about element's details (like id or uuid) but I know that I want to remove the first N elements from the start of it. Is there a way to do it in mongoDB? I know I can fetch the whole document and process it in my own programming language environment but I thought it would be nicer if mongoDB already implemented a way to achieve it atomically by its own query language.

Upvotes: 0

Views: 242

Answers (2)

omidh
omidh

Reputation: 2822

What @turivishal metioned is true however it only works if array's size is always 4. For it to work for all array sizes we should consider size of the array in aggregation, so:

let n = 2;
db.collection.update({},
[
  {
    $set: {
      arr: {
        $slice: [
          "$arr",
          {
            $subtract: [
              -n,
              {
                $size: "$arr"
              }
            ]
          },
          
        ]
      }
    }
  }
])

Playground

Upvotes: 0

turivishal
turivishal

Reputation: 36104

There is a $pop operator to remove a single element from the array either from top or bottom position,

and there is a closed jira support request SERVER-4798 regarding multiple pop operations, but in comment they have suggested to use update with aggregation pipeline option.

So you can try update with aggregation pipeline starting from MongoDB 4.2,

  • $slice, pass negative number and it will slice elements from 0 index
let n = 2;
db.collection.updateOne(
  {}, // your query
  [{
    $set: { arr: { $slice: ["$arr", -n] } }
  }]
)

Playground

Upvotes: 1

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