Find the longest item in array of strings in MongoDB

Question

A have a collection Products in MongoDB:

{
  "name": "My Product 01",
  "description": "This is an excelent product", 
  "tags": [
    "AA",
    "BBBB",
    "C",
    "DDDDDDDDDDD"
  ]  
}

How can I find the single longest occurrence if an item in the whole collection? (if I had only this object, the longest occurrence would be "DDDDDDDDDDD" with 11 characters).

Answer 1

$unwind to tags level. Use $strLenCP to compute the length. Use $setWindowFields to $rank the length. $match rank: 1 to select the one(s) with longest length.

db.Products.aggregate([
  {
    "$unwind": "$tags"
  },
  {
    "$set": {
      "length": {
        "$strLenCP": "$tags"
      }
    }
  },
  {
    "$setWindowFields": {
      "partitionBy": null,
      "sortBy": {
        "length": -1
      },
      "output": {
        "rank": {
          $rank: {}
        }
      }
    }
  },
  {
    "$match": {
      rank: 1
    }
  }
])

Mongo Playground