[mongodb] String field value length in mongoDB

For MongoDB 3.6 and newer:

The $expr operator allows the use of aggregation expressions within the query language, thus you can leverage the use of $strLenCP operator to check the length of the string as follows:

db.usercollection.find({ 
    "name": { "$exists": true },
    "$expr": { "$gt": [ { "$strLenCP": "$name" }, 40 ] } 
})

For MongoDB 3.4 and newer:

You can also use the aggregation framework with the $redact pipeline operator that allows you to proccess the logical condition with the $cond operator and uses the special operations $$KEEP to "keep" the document where the logical condition is true or $$PRUNE to "remove" the document where the condition was false.

This operation is similar to having a $project pipeline that selects the fields in the collection and creates a new field that holds the result from the logical condition query and then a subsequent $match, except that $redact uses a single pipeline stage which is more efficient.

As for the logical condition, there are String Aggregation Operators that you can use $strLenCP operator to check the length of the string. If the length is $gt a specified value, then this is a true match and the document is "kept". Otherwise it is "pruned" and discarded.

Consider running the following aggregate operation which demonstrates the above concept:

db.usercollection.aggregate([
    { "$match": { "name": { "$exists": true } } },
    {
        "$redact": {
            "$cond": [
                { "$gt": [ { "$strLenCP": "$name" }, 40] },
                "$$KEEP",
                "$$PRUNE"
            ]
        }
    },
    { "$limit": 2 }
])

If using $where, try your query without the enclosing brackets:

db.usercollection.find({$where: "this.name.length > 40"}).limit(2);

A better query would be to to check for the field's existence and then check the length:

db.usercollection.find({name: {$type: 2}, $where: "this.name.length > 40"}).limit(2); 

or:

db.usercollection.find({name: {$exists: true}, $where: "this.name.length > 
40"}).limit(2); 

MongoDB evaluates non-$where query operations before $where expressions and non-$where query statements may use an index. A much better performance is to store the length of the string as another field and then you can index or search on it; applying $where will be much slower compared to that. It's recommended to use JavaScript expressions and the $where operator as a last resort when you can't structure the data in any other way, or when you are dealing with a small subset of data.

A different and faster approach that avoids the use of the $where operator is the $regex operator. Consider the following pattern which searches for

db.usercollection.find({"name": {"$type": 2, "$regex": /^.{41,}$/}}).limit(2); 

Note - From the docs:

If an index exists for the field, then MongoDB matches the regular expression against the values in the index, which can be faster than a collection scan. Further optimization can occur if the regular expression is a “prefix expression”, which means that all potential matches start with the same string. This allows MongoDB to construct a “range” from that prefix and only match against those values from the index that fall within that range.

A regular expression is a “prefix expression” if it starts with a caret (^) or a left anchor (\A), followed by a string of simple symbols. For example, the regex /^abc.*/ will be optimized by matching only against the values from the index that start with abc.

Additionally, while /^a/, /^a.*/, and /^a.*$/ match equivalent strings, they have different performance characteristics. All of these expressions use an index if an appropriate index exists; however, /^a.*/, and /^a.*$/ are slower. /^a/ can stop scanning after matching the prefix.