$where¶
-
$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 eitherthis
orobj
.Warning
- Do not write to the database within the
$where
JavaScript function. $where
evaluates JavaScript and cannot take advantage of indexes. Therefore, query performance improves when you express your query using the standard MongoDB operators (e.g.,$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 table scan.
Consider the following examples:
Additionally, if the query consists only of the
$where
operator, you can pass in just the JavaScript expression or JavaScript functions, as in the following examples:You can include both the standard MongoDB operators and the
$where
operator in your query, as in the following examples: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-
$where
query statements may use an index.
Note
Changed in version 2.4.
In MongoDB 2.4,
map-reduce operations
, thegroup
command, and$where
operator expressions cannot access certain global functions or properties, such asdb
, that are available in themongo
shell.When upgrading to MongoDB 2.4, you will need to refactor your code if your
map-reduce operations
,group
commands, or$where
operator expressions include any global shell functions or properties that are no longer available, such asdb
.The following JavaScript functions and properties are available to
map-reduce operations
, thegroup
command, and$where
operator expressions in MongoDB 2.4: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()
- Do not write to the database within the