[SOLVED] String field value length in mongoDB

String field value length in mongoDB

The data type of the field is String. I would like to fetch the data where character length of field name is greater than 40.

I tried these queries but returning error. 1.

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

output :error: {
    "$err" : "TypeError: Cannot read property 'length' of undefined near '40)' ",
    "code" : 16722
}

this is working in 2.4.9 But my version is 2.6.5

Solution

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.