db.collection.findOne()

Client Disconnection客户端断开连接

Starting in MongoDB 4.2, if the client that issued db.collection.findOne() disconnects before the operation completes, MongoDB marks db.collection.findOne() for termination using killOp.从MongoDB 4.2开始，如果发出db.collection.findOne()的客户端在操作完成前断开连接，MongoDB会使用killOp标记db.collection.findOne()终止。

Projection投影

The projection parameter determines which fields are returned in the matching documents. The projection parameter takes a document of the following form:projection参数确定在匹配的文档中返回哪些字段。projection参数采用以下形式的文档：

{ field1: <value>, field2: <value> ... }

With Empty Query Specification带有空查询规范

The following operation returns a single document from the bios collection:以下操作从bios集合返回单个文档：

db.bios.findOne()

With a Query Specification使用查询规范

The following operation returns the first matching document from the bios collection where either the field first in the embedded document name starts with the letter G or where the field birth is less than new Date('01/01/1945'):以下操作返回bios集合中的first匹配文档，其中嵌入文档名称中第一个字段以字母G开头，或者字段birth小于new Date('01/01/1945')：

db.bios.findOne(
   {
     $or: [
            { 'name.first' : /^G/ },
            { birth: { $lt: new Date('01/01/1945') } }
          ]
   }
)

With a Projection使用投影

The projection parameter specifies which fields to return. projection参数指定要返回的字段。The parameter contains either include or exclude specifications, not both, unless the exclude is for the _id field.参数包含include或exclude规范，而不是两者都包含，除非exclude用于_id字段。

db.bios.findOne(
    { },
    { name: 1, contribs: 1 }
)

db.bios.findOne(
   { contribs: 'OOP' },
   { _id: 0, 'name.first': 0, birth: 0 }
)

The findOne Result DocumentfindOne结果文档

You cannot apply cursor methods to the result of findOne() because a single document is returned. 不能将游标方法应用于findOne()的结果，因为只返回了一个文档。You have access to the document directly:您可以直接访问文档：

var myDocument = db.bios.findOne();

if (myDocument) {
   var myName = myDocument.name;

   print (tojson(myName));
}

Use Variables in let Option在let选项中使用变量

You can specify query options to modify query behavior and indicate how results are returned.您可以指定查询选项来修改查询行为并指示返回结果的方式。

For example, to define variables that you can access elsewhere in the findOne method, use the let option. 例如，要定义可以在findOne方法的其他地方访问的变量，请使用let选项。To filter results using a variable, you must access the variable within the $expr operator.若要使用变量筛选结果，必须访问$expr运算符中的变量。

Create a collection cakeFlavors:创建一个集合cakeFlavors：

db.cakeFlavors.insertMany( [
   { _id: 1, flavor: "chocolate" },
   { _id: 2, flavor: "strawberry" },
   { _id: 3, flavor: "cherry" }
] )

The following example defines a targetFlavor variable in let and uses the variable to retrieve the chocolate cake flavor:以下示例在let中定义了targetFlavor变量，并使用该变量检索巧克力蛋糕的风味：

db.cakeFlavors.findOne(
   { $expr: { $eq: [ "$flavor", "$$targetFlavor" ] } },
   { _id: 0 },
   { let : { targetFlavor: "chocolate" }
} )

Output:输出：

{ flavor: 'chocolate' }

To see all available query options, see FindOptions.要查看所有可用的查询选项，请参阅FindOptions。