$replaceOne (aggregation)

Operator Fields运算符字段

$replaceOne and Null Values和Null值

If input or find refer to a field that is missing, they return null.如果input或find引用了缺失的字段，则返回null。

If any one of input, find, or replacement evaluates to a null, the entire $replaceOne expression evaluates to null:如果input、find或replacement中的任何一个的计算结果为null，则整个$replaceOne表达式的计算结果将为null：

$replaceOne and Collation和排序规则

String matching for all $replaceOne expressions is always case-sensitive and diacritic-sensitive. 所有$replaceOne表达式的字符串匹配始终区分大小写和变音符号。Any collation configured is ignored when performing string comparisons with $replaceOne.与$replaceOne执行字符串比较时，将忽略任何配置的collation。

For example, create a sample collection with collation strength 1:例如，创建排序规则强度为1:

db.createCollection( "myColl", { collation: { locale: "fr", strength: 1 } } )

A collation strength of 1 compares base character only and ignores other differences such as case and diacritics.排序规则强度为1时，只比较基本字符，而忽略其他差异，如大小写和变音符号。

Next, insert three example documents:接下来，插入三个示例文档：

db.myColl.insertMany([
   { _id: 1, name: "cafe" },
   { _id: 2, name: "Cafe" },
   { _id: 3, name: "café" }
])

The following $replaceOne operation tries to find and replace the first instance of "Cafe" in the name field:以下$replaceOne操作尝试在name字段中查找并替换“Cafe”的第一个实例：

db.myColl.aggregate([
  {
    $addFields:
      {
        resultObject: { $replaceOne: { input: "$name", find: "Cafe", replacement: "CAFE" } }
      }
  }
])

Because $replaceOne ignores the collation configured for this collection, the operation only matches the instance of "Cafe" in document 2:由于$replaceOne忽略为此集合配置的排序规则，因此该操作仅与文档2中的“Cafe”实例匹配：

{ "_id" : 1, "name" : "cafe", "resultObject" : "cafe" }
{ "_id" : 2, "name" : "Cafe", "resultObject" : "CAFE" }
{ "_id" : 3, "name" : "café", "resultObject" : "café" }

Operators which respect collation, such as $match, would match all three documents when performing a string comparison against "Cafe" due to this collection's collation strength of 1.尊重排序规则的运算符（如$match）在与“Cafe”进行字符串比较时会匹配所有三个文档，因为该集合的排序规则强度为1。

$replaceOne and Unicode Normalization和Unicode规范化

The $replaceOne aggregation expression does not perform any unicode normalization. $replaceOne聚合表达式不执行任何unicode规范化。This means that string matching for all $replaceOne expressions will consider the number of code points used to represent a character in unicode when attempting a match.这意味着，在尝试匹配时，所有$replaceOne表达式的字符串匹配都将考虑用于表示unicode中字符的代码点数。

For example, the character é can be represented in unicode using either one code point or two:例如，字符é可以使用一个或两个代码点在unicode中表示：

Using $replaceOne with a find string where the character é is represented in unicode with one code point will not match any instance of é that uses two code points in the input string.将$replaceOne与find字符串（其中字符é在unicode中用一个代码点表示）一起使用，将不会与input字符串中使用两个代码点的任何é实例匹配。

The following table shows whether a match occurs for a find string of "café" when compared to input strings where é is represented by either one code point or two. 下表显示了与由一个或两个代码点表示的find字符串é相比，find字符串“café”是否匹配。The find string in this example uses one code point to represent the é character:本例中的find字符串使用一个代码点来表示字符é：

Because $replaceOne does not perform any unicode normalization, only the first string comparison matches, where both the find and input strings use one code point to represent é.由于$replaceOne不执行任何unicode规范化，因此只有第一个字符串比较匹配，其中find字符串和input字符串都使用一个代码点来表示é。