Select string length in mongodb

Question

How do you query mongodb to find the length of a particular string/text field?

And how would you find the max length of a query set?

Answer 1

Starting Mongo 3.4, the $strLenCP aggregation operator can be used to get a string's length:

// { a: "Hello World" }
// { a: "42" }
// { a: "Hello World!" }
db.collection.aggregate([{ $addFields: { length: { $strLenCP: "$a" } } }])
// { a: "Hello World",  length: 11 }
// { a: "42",           length: 2 }
// { a: "Hello World!", length: 12 }

and in order to get the max length from all documents via a $group/$max stage:

db.collection.aggregate([
  { $group: { _id: null, longest: { $max: { $strLenCP: "$a" } } } }
])
// { "_id" : null, longest: 12 }

Answer 2

how about use regular expression instead.

> db.apps.find({$where:"(this.id.length gt 6) && (this.id.length lt 15) " } ).count();
2548
> db.apps.find({$where:" (this.id.length gt 6) && (this.id.length lt 15) " } ).explain();
{
    "cursor" : "BasicCursor",
    "isMultiKey" : false,
    "n" : 2548,
    "nscannedObjects" : 88736,
    "nscanned" : 88736,
    "nscannedObjectsAllPlans" : 88736,
    "nscannedAllPlans" : 88736,
    "scanAndOrder" : false,
    "indexOnly" : false,
    "nYields" : 1,
    "nChunkSkips" : 0,
    "millis" : 1523,
    "indexBounds" : {

    },
    "server" : "shuhaimac.local:27017"
}

> db.apps.find({id:/\w{7,16}/i}).count();
2548
> db.apps.find({id:/\w{7,16}/i}).explain();
{
    "cursor" : "BtreeCursor id_1 multi",
    "isMultiKey" : false,
    "n" : 2548,
    "nscannedObjects" : 2548,
    "nscanned" : 88736,
    "nscannedObjectsAllPlans" : 2548,
    "nscannedAllPlans" : 88736,
    "scanAndOrder" : false,
    "indexOnly" : false,
    "nYields" : 0,
    "nChunkSkips" : 0,
    "millis" : 122,
    "indexBounds" : {
        "id" : [
            [
                "",
                {

                }
            ],
            [
                /\w{7,16}/i,
                /\w{7,16}/i
            ]
        ]
    },
    "server" : "shuhaimac.local:27017"
}