问题描述
该字段的数据类型为字符串.我想获取字段名称的字符长度大于40的数据.
我尝试了这些查询,但返回错误.1.
db.usercollection.find(
{$where: "(this.name.length > 40)"}
).limit(2);
output :error: {
"$err" : "TypeError: Cannot read property 'length' of undefined near '40)' ",
"code" : 16722
}
这在2.4.9中有效,但我的版本是2.6.5
对于MongoDB 3.6及更高版本:
$expr
运算符允许在查询语言内使用聚合表达式,因此您可以利用 $strLenCP
运算符,如下所示检查字符串的长度:
db.usercollection.find({
"name": { "$exists": true },
"$expr": { "$gt": [ { "$strLenCP": "$name" }, 40 ] }
})
对于MongoDB 3.4及更高版本:
您还可以将聚合框架与 $redact
管道运算符,可让您使用 $cond
运算符,并使用特殊操作 $$KEEP
以保留"逻辑条件为true或 $$PRUNE
以删除"条件为假的文档
此操作类似于使用 $project
管道,该管道选择集合中的字段并创建一个新字段,该字段保存逻辑条件查询的结果,然后保存随后的 $match
,除了 $redact
使用单个管道阶段,效率更高. /p>
对于逻辑条件,有字符串聚合运算符您可以使用 $strLenCP
运算符以检查字符串的长度.如果长度为 $gt
指定的值,那么这是一个真正的匹配,并且文档被保留".否则,将其修剪"并丢弃.
请考虑运行以下汇总操作,以证明上述概念:
db.usercollection.aggregate([
{ "$match": { "name": { "$exists": true } } },
{
"$redact": {
"$cond": [
{ "$gt": [ { "$strLenCP": "$name" }, 40] },
"$$KEEP",
"$$PRUNE"
]
}
},
{ "$limit": 2 }
])
如果使用 $where
> ,请尝试在不带括号的情况下进行查询:
db.usercollection.find({$where: "this.name.length > 40"}).limit(2);
更好的查询方法是检查字段的存在,然后检查长度:
db.usercollection.find({name: {$type: 2}, $where: "this.name.length > 40"}).limit(2);
或:
db.usercollection.find({name: {$exists: true}, $where: "this.name.length >
40"}).limit(2);
MongoDB评估非 $where
在 $where
表达式和非$where
查询语句都可以使用索引.更好的性能是将字符串的长度存储为另一个字段,然后可以对其进行索引或搜索.应用 $where
$where
运算符,当您无法以任何其他方式构造数据或处理数据时,一小部分数据.
一种避免使用 $where
运算符是 $regex
运算符.考虑以下搜索
db.usercollection.find({"name": {"$type": 2, "$regex": /^.{41,}$/}}).limit(2);
注意-从 文档 :
The data type of the field is String. I would like to fetch the data where character length of field name is greater than 40.
I tried these queries but returning error.1.
db.usercollection.find(
{$where: "(this.name.length > 40)"}
).limit(2);
output :error: {
"$err" : "TypeError: Cannot read property 'length' of undefined near '40)' ",
"code" : 16722
}
this is working in 2.4.9 But my version is 2.6.5
For MongoDB 3.6 and newer:
The $expr
operator allows the use of aggregation expressions within the query language, thus you can leverage the use of $strLenCP
operator to check the length of the string as follows:
db.usercollection.find({
"name": { "$exists": true },
"$expr": { "$gt": [ { "$strLenCP": "$name" }, 40 ] }
})
For MongoDB 3.4 and newer:
You can also use the aggregation framework with the $redact
pipeline operator that allows you to proccess the logical condition with the $cond
operator and uses the special operations $$KEEP
to "keep" the document where the logical condition is true or $$PRUNE
to "remove" the document where the condition was false.
This operation is similar to having a $project
pipeline that selects the fields in the collection and creates a new field that holds the result from the logical condition query and then a subsequent $match
, except that $redact
uses a single pipeline stage which is more efficient.
As for the logical condition, there are String Aggregation Operators that you can use $strLenCP
operator to check the length of the string. If the length is $gt
a specified value, then this is a true match and the document is "kept". Otherwise it is "pruned" and discarded.
Consider running the following aggregate operation which demonstrates the above concept:
db.usercollection.aggregate([
{ "$match": { "name": { "$exists": true } } },
{
"$redact": {
"$cond": [
{ "$gt": [ { "$strLenCP": "$name" }, 40] },
"$$KEEP",
"$$PRUNE"
]
}
},
{ "$limit": 2 }
])
If using $where
, try your query without the enclosing brackets:
db.usercollection.find({$where: "this.name.length > 40"}).limit(2);
A better query would be to to check for the field's existence and then check the length:
db.usercollection.find({name: {$type: 2}, $where: "this.name.length > 40"}).limit(2);
or:
db.usercollection.find({name: {$exists: true}, $where: "this.name.length >
40"}).limit(2);
MongoDB evaluates non-$where
query operations before $where
expressions and non-$where
query statements may use an index. A much better performance is to store the length of the string as another field and then you can index or search on it; applying $where
will be much slower compared to that. It's recommended to use JavaScript expressions and the $where
operator as a last resort when you can't structure the data in any other way, or when you are dealing with asmall subset of data.
A different and faster approach that avoids the use of the $where
operator is the $regex
operator. Consider the following pattern which searches for
db.usercollection.find({"name": {"$type": 2, "$regex": /^.{41,}$/}}).limit(2);
Note - From the docs:
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