$replaceOne (aggregation)（聚合）

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 $replaceOne expression evaluates to null:如果input、find或replacement中的任何一个计算结果为null，则整个$replaceOne表达式计算结果为空：

$replaceOne and Collation$replaceOne和排序规则

String matching for $replaceOne expressions is always case-sensitive and diacritic-sensitive. $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 $replaceOne.使用$replaceOne执行字符串比较时，将忽略配置的任何排序规则。

For example, create a sample collection with collation strength 1:例如，创建排序强度为1的样本集合：

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

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

Next, insert example documents:接下来，插入示例文档：

db.restaurants.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操作尝试查找并替换name字段中的“Cafe”的第一个实例：

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

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

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

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

$replaceOne and Unicode NormalizationUnicode标准化

The $replaceOne aggregation expression does not perform any unicode normalization. This means that string matching for all $replaceOne expressions will consider the number of code points used to represent a character in unicode when attempting a match.$replaceOne聚合表达式不执行任何unicode规范化。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以unicode表示字符的码点数。

For example, the character é can be represented in unicode using either one code point or two:例如，字符é可以用一个或两个码位以unicode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字符串一起使用，其中字符é以unicode表示，只有一个码点，这将与input字符串中使用两个码点的任何é实例都不匹配。

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input字符串相比，find字符串“café”是否匹配，其中é由一个或两个代码点表示。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é字符：

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

Replace Using a String使用字符串替换

The following example replaces the first instance of "blue paint" in the item field with "red paint":以下示例将item字段中的“蓝色油漆”的第一个实例替换为“红色油漆”：

db.inventory.aggregate([
   {
     $project:
      {
         item: { $replaceOne: { input: "$item", find: "blue paint", replacement: "red paint" } }
      }
   }
])

The operation returns the following results:该操作返回以下结果：

{ _id: 1, item: "red paint" }
{ _id: 2, item: "blue and green paint" }
{ _id: 3, item: "red paint with blue paintbrush" }
{ _id: 4, item: "red paint with green paintbrush" }

Note that with document 3, only the first matched instance of "blue paint" is replaced.请注意，在文档3中，只有第一个匹配的“蓝色油漆”实例被替换。

Replace Using Regex使用正则表达式替换

The following example replaces the first instance of "blue" as a whole word in the item field with "navy":以下示例将item字段中“blue”作为一个完整单词的第一个实例替换为“navy”：

db.inventory.aggregate([
   {
     $project:
      {
         item: { $replaceOne: { input: "$item", find: \\bblue\\b, replacement: "navy" } }
      }
   }
]);

The operation returns the following results:该操作返回以下结果：

{ _id: 1, item: "navy paint" }
{ _id: 2, item: "navy and green paint" }
{ _id: 3, item: "navy paint with blue paintbrush" }
{ _id: 4, item: "navy paint with green paintbrush" }

Note that with document 3, only the first matched instance of "blue" is replaced.请注意，在文档3中，只有第一个匹配的“蓝色”实例被替换。