$where¶
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- $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 .
Behavior¶
Restrictions¶
在 2.4 版更改.
In MongoDB 2.4, 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.
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 as db.
The following JavaScript functions and properties are available to map-reduce operations, the group 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()
|
elemMatch¶
在 2.6 版更改.
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.
Examples¶
Consider the following examples:
db.myCollection.find( { $where: "this.credits == this.debits" } );
db.myCollection.find( { $where: "obj.credits == obj.debits" } );
db.myCollection.find( { $where: function() { return (this.credits == this.debits) } } );
db.myCollection.find( { $where: function() { return obj.credits == obj.debits; } } );
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:
db.myCollection.find( "this.credits == this.debits || this.credits > this.debits" );
db.myCollection.find( function() { return (this.credits == this.debits || this.credits > this.debits ) } );
You can include both the standard MongoDB operators and the $where operator in your query, as in the following examples:
db.myCollection.find( { active: true, $where: "this.credits - this.debits < 0" } );
db.myCollection.find( { active: true, $where: function() { return obj.credits - obj.debits < 0; } } );