mongoDB 中的字符串字段值长度

Question

该字段的数据类型为String。我想获取字段名称字符长度大于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
}

这适用于 2.4.9 但我的版本是 2.6.5

Answer 1

对于 MongoDB 3.6 和更新版本：

$expr operator allows the use of aggregation expressions within the query language, thus you can leverage the use of $strLenCP运算符检查字符串长度如下：

db.usercollection.find({ 
    name: { $exists: true },
    $expr: { $gt: [{ $strLenCP: '$name' }, 40] } 
})

对于 MongoDB 3.4 和更新版本：

您还可以使用带有 $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 的聚合框架来“删除”条件为假的文档。

此操作类似于让 $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 使用效率更高的单个流水线阶段。

至于逻辑条件，有String Aggregation Operators that you can use $strLenCP operator to check the length of the string. If the length is $gt个指定值，则此为真匹配，文档被“保留”。否则它会被“修剪”并丢弃。

考虑运行以下证明上述概念的聚合操作：

db.usercollection.aggregate([
    { $match: { name: { $exists: true } } },
    { $redact: {
         $cond: [
            { $gt: [ { $strLenCP: "$name" }, 40] },
            "$$KEEP",
            "$$PRUNE"
        ]
    } },
    { $limit: 2 }
])

如果使用 $where，请尝试不带括号的查询：

db.usercollection.find({ $where: "this.name.length > 40" }).limit(2);

更好的查询是检查字段是否存在，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 在 $where 表达式和非 $where 查询语句可以使用索引之前评估非 $where 查询操作。更好的性能是将字符串的长度存储为另一个字段，然后您可以对其进行索引或搜索；与此相比，应用 $where 会慢得多。当您无法以任何其他方式构建数据时，或者当您处理数据的小子集。

避免使用 $where 运算符的另一种更快的方法是 $regex 运算符。考虑以下搜索

的模式

db.usercollection.find({"name": {"$type": 2, "$regex": /^.{41,}$/}}).limit(2);

注意 - 来自docs:

If an index exists for the field, then MongoDB matches the regular expression against the values in the index, which can be faster than a collection scan. Further optimization can occur if the regular expression is a “prefix expression”, which means that all potential matches start with the same string. This allows MongoDB to construct a “range” from that prefix and only match against those values from the index that fall within that range.

A regular expression is a “prefix expression” if it starts with a caret (^) or a left anchor (\A), followed by a string of simple symbols. For example, the regex /^abc.*/ will be optimized by matching only against the values from the index that start with abc.

Additionally, while /^a/, /^a.*/, and /^a.*$/ match equivalent strings, they have different performance characteristics. All of these expressions use an index if an appropriate index exists; however, /^a.*/, and /^a.*$/ are slower. /^a/ can stop scanning after matching the prefix.

Answer 2

我有过类似的情况，但在我的情况下，字符串不是一级属性。它在 object 内。在这里我找不到合适的答案。所以我想与大家分享我的解决方案（希望这会帮助任何有类似问题的人）。

Parent Collection 

{
"Child":
{
"name":"Random Name",
"Age:"09"
}
}

例如：如果我们只需要获取 child 名称长度超过 10 个字符的集合。

 db.getCollection('Parent').find({$where: function() { 
for (var field in this.Child.name) { 
    if (this.Child.name.length > 10) 
        return true;

}
}})

Answer 3

这是 mongodb 中的一种方法，您可以实现此目的。

db.usercollection.find({ $where: 'this.name.length < 4' })

Answer 4

如果文档太多，使用 $where 和 $expr 的查询会很慢。

使用$regex比$where、$expr快得多。

db.usercollection.find({ 
  "name": /^[\s\S]{40,}$/, // name.length >= 40
})

or 

db.usercollection.find({ 
  "name": { "$regex": "^[\s\S]{40,}$" }, // name.length >= 40
})

本查询与

同义

db.usercollection.find({ 
  "$where": "this.name && this.name.length >= 40",
})

or

db.usercollection.find({ 
    "name": { "$exists": true },
    "$expr": { "$gte": [ { "$strLenCP": "$name" }, 40 ] } 
})

我针对 collection.

测试了每个查询

# find
$where: 10529.359ms
$expr: 5305.801ms
$regex: 2516.124ms

# count
$where: 10872.006ms
$expr: 2630.155ms
$regex: 158.066ms

Answer 5

此查询将给出字段值和长度：

db.usercollection.aggregate([
{
    $project: {
        "name": 1,
        "length": { $strLenCP: "$name" }
    }} ])

mongoDB 中的字符串字段值长度

String field value length in mongoDB

mongodb

field

string-length