db.collection.findOne() (mongosh method方法)

Client Disconnection客户端断开连接

If the client that issued db.collection.findOne() disconnects before the operation completes, MongoDB marks db.collection.findOne() for termination using killOp.如果发出db.collection.findOne()的客户端在操作完成之前断开连接，MongoDB将使用killOp标记db.collection.findOne()以终止。

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参数决定了在匹配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使用空查询规范

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集合中的第一个匹配文档，其中嵌入文档name中的first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带投影

The projection parameter specifies which fields to return. The parameter contains either include or exclude specifications, not both, unless the exclude is for the _id field.projection参数指定要返回哪些字段。该参数包含纳入或排除规范，而不是两者都包含，除非排除是针对_id字段的。

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

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

With an Option有一个选项

The following operation uses the sort option to return the first matching document from the sorted bios collection. In this example, the collection is sorted by birth in ascending order.以下操作使用sort选项从已排序的bios集合中返回第一个匹配的文档。在这个例子中，集合按birth顺序升序排列。

db.bios.findOne(
   { },
   { },
   { sort: { birth: 1 } }
)

The findOne Result DocumentfindOne结果文档

You cannot apply cursor methods to the result of findOne() because a single document is returned. You have access to the document directly:您不能将游标方法应用于findOne()的结果，因为返回的是单个文档。您可以直接访问该文档：

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，并指示如何返回结果。

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 $expr operator.要使用变量筛选结果，您必须在$expr运算符中访问该变量。

Create a collection cakeFlavors:创建一个系列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。