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mongodb / v3.6 / reference / reference-operator-query-where.html

$where

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Definition

$where

Use the $where operator to pass either a string containing a JavaScript expression or a full JavaScript function to the query system. The $where provides greater flexibility, but requires that the database processes the JavaScript expression or function for each document in the collection. Reference the document in the JavaScript expression or function using either this or obj .

Important

Changed in version 3.6: The $expr operator allows the use of aggregation expressions within the query language. $expr is faster than $where because it does not execute JavaScript and should be preferred where possible.

Behavior

Restrictions

map-reduce operations, the group command, and $where operator expressions cannot access certain global functions or properties, such as db, that are available in the mongo shell.

The following JavaScript functions and properties are available to map-reduce operations, the group command, and $where operator expressions:

Available Properties Available Functions  
args
MaxKey
MinKey
assert()
BinData()
DBPointer()
DBRef()
doassert()
emit()
gc()
HexData()
hex_md5()
isNumber()
isObject()
ISODate()
isString()
Map()
MD5()
NumberInt()
NumberLong()
ObjectId()
print()
printjson()
printjsononeline()
sleep()
Timestamp()
tojson()
tojsononeline()
tojsonObject()
UUID()
version()

elemMatch

Only apply the $where query operator to top-level documents. The $where query operator will not work inside a nested document, for instance, in an $elemMatch query.

Considerations

  • Do not use global variables.
  • $where evaluates JavaScript and cannot take advantage of indexes. Therefore, query performance improves when you express your query using the standard MongoDB operators (e.g., $gt, $in).
  • In general, you should use $where only when you can’t express your query using another operator. If you must use $where, try to include at least one other standard query operator to filter the result set. Using $where alone requires a collection scan.

Using normal non-$where query statements provides the following performance advantages:

  • MongoDB will evaluate non-$where components of query before $where statements. If the non-$where statements match no documents, MongoDB will not perform any query evaluation using $where.
  • The non-$where query statements may use an index.

Example

Consider the following documents in the players collection:

{
   _id: 12378,
   name: "Steve",
   username: "steveisawesome",
   first_login: "2017-01-01"
}
{
   _id: 2,
   name: "Anya",
   username: "anya",
   first_login: "2001-02-02"
}

The following example uses $where and the hex_md5() JavaScript function to compare the value of the name field to an MD5 hash and returns any matching document.

db.players.find( { $where: function() {
   return (hex_md5(this.name) == "9b53e667f30cd329dca1ec9e6a83e994")
} } );

The operation returns the following result:

{
   "_id" : 2,
   "name" : "Anya",
   "username" : "anya",
   "first_login" : "2001-02-02"
}