$where¶
On this page
-
$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
.
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:
Examples¶
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: