cursor.returnKey() (mongosh method方法)

Definition定义

cursor.returnKey()

Important

mongosh Method方法

This page documents a mongosh method. This is not the documentation for a language-specific driver, such as Node.js.本页记录了一种mongosh方法。这不是针对特定语言驱动程序（如Node.js）的文档。

For MongoDB API drivers, refer to the language-specific MongoDB driver documentation.有关MongoDB API驱动程序，请参阅特定语言的MongoDB驱动程序文档。

Tip

$meta supports the keyword "indexKey" to return index key metadata if an index is used. 支持键"indexKey"在使用索引时返回索引键元数据。The use of { $meta: "indexKey" } is preferred over cursor.returnKey().使用{ $meta: "indexKey" }比cursor.returnKey()更可取。

Modifies the cursor to return index keys rather than the documents.修改游标以返回索引键而不是文档。

The cursor.returnKey() has the following form:cursorreturnKey()具有以下形式：

cursor.returnKey()

Returns:返回	The cursor that returnKey() is attached to with a modified result set. This allows for additional cursor modifiers to be chained.返回returnKey()的游标附加了修改后的结果集。这允许链接其他游标修改器。

Compatibility兼容性

This method is available in deployments hosted in the following environments:此方法在以下环境中托管的部署中可用：

• MongoDB Atlas: The fully managed service for MongoDB deployments in the cloud：云中MongoDB部署的完全托管服务

• MongoDB Enterprise: The subscription-based, self-managed version of MongoDB：MongoDB的基于订阅的自我管理版本
• MongoDB Community: The source-available, free-to-use, and self-managed version of MongoDB：MongoDB的源代码可用、免费使用和自我管理版本

Behavior行为

If the query does not use an index to perform the read operation, the cursor returns empty documents.如果查询不使用索引来执行读取操作，则游标将返回空文档。

Example示例

The restaurants collection contains documents with the following schema:restaurants集合包含具有以下模式的文档：

db.restaurants.insertOne(
   {
      _id: ObjectId("564f3a35b385149fc7e3fab9"),
      address: {
         building: "2780",
         coord: [
            -73.98241999999999,
            40.579505
         ],
         street: "Stillwell Avenue",
         zipcode: "11224"
      },
      borough: "Brooklyn",
      cuisine: "American ",
      grades: [
         {
            date: ISODate("2014-06-10T00:00:00Z"),
            grade: "A",
            score: 5
         },
         {
            date: ISODate("2013-06-05T00:00:00Z"),
            grade: "A",
            score: 7
         }
      ],
      name: "Riviera Caterer",
      restaurant_id: "40356018"
   }
)

The collection has two indexes in addition to the default _id index:除了默认的_id索引外，该集合还有两个索引：

{
   "v" : 1,
   "key" : {
      "_id" : 1
   },
   "name" : "_id_",
   "ns" : "guidebook.restaurant"
},
{
   "v" : 1,
   "key" : {
      "cuisine" : 1
   },
   "name" : "cuisine_1",
   "ns" : "guidebook.restaurant"
},
{
   "v" : 1,
   "key" : {
      "_fts" : "text",
      "_ftsx" : 1
   },
   "name" : "name_text",
   "ns" : "guidebook.restaurant",
   "weights" : {
      "name" : 1
   },
   "default_language" : "english",
   "language_override" : "language",
   "textIndexVersion" : 3
}

The following code uses the cursor.returnKey() method to return only the indexed fields used for executing the query:以下代码使用cursor.returnKey()方法仅返回用于执行查询的索引字段：

var csr = db.restaurant.find( { "cuisine" : "Japanese" } )
csr.returnKey()

This returns the following:这将返回以下内容：

{ "cuisine" : "Japanese" }
{ "cuisine" : "Japanese" }
{ "cuisine" : "Japanese" }
{ "cuisine" : "Japanese" }
...