Dollar-Prefixed Field Names以美元为前缀的字段名

Insert Operations插入操作

Dollar ($) prefixed fields are permitted as top level and nested field names for inserts.允许使用美元（$）前缀字段作为插入的顶级和嵌套字段名。

db.sales.insertOne( {
   "$price": 50.00,
   "quantity": 30
} )

Dollar ($) prefixed fields are permitted on inserts using otherwise reserved words. 使用保留字插入时，允许使用美元（$）前缀字段。Operator names like $inc can be used as field names as well as words like id, db, and ref.像$inc这样的运算符名称既可以用作字段名称，也可以用作id、db和ref这样的单词。

db.books.insertOne( {
   "$id": "h1961-01",
   "location": {
      "$db": "novels",
      "$ref": "2007042768",
      "$inc": true
} } )

An update which creates a new document during an upsert is treated as an insert rather than an update for field name validation. Upserts can accept dollar ($) prefixed fields. 在upsert期间创建新文档的更新被视为insert，而不是字段名验证的update。upsert可以接受以美元（$）为前缀的字段。However, upserts are a special case and similar update operations may cause an error if the match portion of the update selects an existing document.然而，upsert是一种特殊情况，如果更新的match部分选择了现有文档，类似的更新操作可能会导致错误。

This code sample has upsert: true so it will insert a new document if the collection doesn't already contain a document that matches the query term, { "date": "2021-07-07" }. 此代码示例具有upsert: true，因此如果集合中尚未包含与查询词{ "date": "2021-07-07" }匹配的文档，它将插入一个新文档。If this sample code matches an existing document, the update will fail since $hotel is dollar ($) prefixed.如果此示例代码与现有文档匹配，则更新将失败，因为$hotel的前缀是美元（$）。

db.expenses.updateOne(
   { "date": "2021-07-07" },
   { $set: {
      "phone": 25.17,
      "$hotel": 320.10
   } },
   { upsert: true }
)

Document Replacing Updates文档替换更新

Update operators either replace existing fields with new documents or else modify those fields. In cases where the update performs a replacement, dollar ($) prefixed fields are not permitted as top level field names.更新运算符可以用新文档替换现有字段，也可以修改这些字段。在更新执行替换的情况下，不允许以美元（$）为前缀的字段作为顶级字段名。

Consider a document like考虑以下文档

{
   "_id": "E123",
   "address": {
      "$number": 123,
      "$street": "Elm Road"
   },
   "$rooms": {
      "br": 2,
      "bath": 1
   }
}

You could use an update operator that replaces an existing document to modify the address.$street field but you could not update the $rooms field that way.您可以使用替换现有文档的更新运算符来修改address.$street字段，但您无法以这种方式更新$rooms字段。

db.housing.updateOne(
   { "_id": "E123" },
   { $set: { "address.$street": "Elm Ave" } }
)

Use $setField as part of an aggregation pipeline to update top level dollar ($) prefixed fields like $rooms.使用$setField作为聚合管道的一部分来更新顶级美元（$）前缀字段，如$rooms。

Document Modifying Updates文档修改更新

When an update modifies, rather than replaces, existing document fields, dollar ($) prefixed fields can be top level field names. Subfields can be accessed directly, but you need a helper method to access the top level fields.当更新修改而不是替换现有文档字段时，美元（$）前缀字段可以是顶级字段名。子字段可以直接访问，但您需要一个辅助方法来访问顶级字段。

Consider a collection with documents like this inventory record:考虑一个包含此类inventory记录文档的集合：

{
   _id: ObjectId("610023ad7d58ecda39b8d161"),
   "part": "AB305",
   "$bin": 200,
   "quantity": 100,
   "pricing": { sale: true, "$discount": 60 }
}

The pricing.$discount subfield can be queried directly.pricing.$discount子字段可以直接查询。

db.inventory.findAndModify( {
    query: { "part": { $eq: "AB305" } },
    update: { $inc: { "pricing.$discount": 10 } }
} )

Use $getField and $literal to access the value of the top level $bin field.

db.inventory.findAndModify( {
   query: { $expr: {
      $eq: [ { $getField: { $literal: "$bin" } }, 200 ]
   } },
   update: { $inc: { "quantity": 10 } }
} )

Updates Using Aggregation Pipelines使用聚合管道进行更新

Use $setField, $getField, and $literal in the $replaceWith stage to modify dollar ($) prefixed fields in an aggregation pipeline.在$replaceWith阶段中使用$setField、$getField和$literal来修改聚合管道中以美元（$）为前缀的字段。

Consider a collection of school records like:考虑一组学校记录，如：

db.school.insertOne (
{
   "_id": 100001,
   "$term": "fall",
   "registered": true,
   "grade": 4
} )

Create a new collection for the spring semester using a pipeline to update the dollar ($) prefixed $term field.使用管道更新以美元（$）为前缀的$term字段，为春季学期创建一个新的集合。

db.school.aggregate( [
   { $match: { "registered": true } },
   { $replaceWith: {
      $setField: {
         field: { $literal: "$term" },
         input: "$$ROOT",
         value: "spring"
   } } },
   { $out: "spring2022" }
] )