$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 eitherthis
orobj
.
Note
Starting in version 4.2.1, MongoDB deprecates the use of BSON
type JavaScript code with scope (BSON type 15) for the $where
operator. Using
JavaScript code with scope with $where
will print a warning
to the log. Instead, use BSON type String (BSON type 2) or BSON type JavaScript
(BSON type 13).
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
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
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: