Specify Which Fields to Return指定要返回的字段

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Use a projection to control which fields appear in the documents returned by read operations. 使用投影控制哪些字段出现在读取操作返回的文档中。Many requests only require certain fields, so projections can help you limit unnecessary network bandwidth usage. 许多请求只需要某些字段，因此预测可以帮助您限制不必要的网络带宽使用。Projections work in two ways:预测工作有两种方式：

  • Explicitly include fields with a value of 1. 显式包含值为1的字段。This has the side-effect of implicitly excluding all unspecified fields.这样做的副作用是隐式排除所有未指定的字段。
  • Implicitly exclude fields with a value of 0. 隐式排除值为0的字段。This has the side-effect of implicitly including all unspecified fields.这样做的副作用是隐式包含所有未指定的字段。

These two methods of projection are mutually exclusive: if you explicitly include fields, you cannot explicitly exclude fields, and vice versa.这两种投影方法是互斥的：如果显式包含字段，则不能显式排除字段，反之亦然。

Consider the following collection containing documents that describe varieties of fruit:考虑下面的包含描述水果品种的文件：

[
  { "_id": 1, "name": "apples", "qty": 5, "rating": 3 },
  { "_id": 2, "name": "bananas", "qty": 7, "rating": 1 },
  { "_id": 3, "name": "oranges", "qty": 6, "rating": 2 },
  { "_id": 4, "name": "avocados", "qty": 3, "rating": 5 },
]
Note

Your query operation may return a reference to a cursor that contains matching documents. 查询操作可能返回对包含匹配文档的游标的引用。To learn how to examine data stored in the cursor, see the Cursor Fundamentals page.要了解如何检查游标中存储的数据，请参阅游标基础知识页面。

In the following query, pass the projection to only return the name field of each document:在以下查询中，传递投影以仅返回每个文档的name字段：

// return only* the name field
const projection = { name: 1 };
const cursor = collection.find().project(projection);
await cursor.forEach(console.dir);

The projection document specifies a value of 1 for name to indicate that the read operation result should include the name field of each returned document. 投影文档为name指定值1，以指示读取操作结果应包括每个返回文档的name字段。As a result, this projection implicitly excludes the qty and rating fields. 因此，该投影隐式排除了qty和rating字段。Passing this projection to find() with an empty query document and no sort document yields the following results:将此投影传递给带有空查询文档且没有排序文档的find()，将产生以下结果：

{ "_id": 1, "name": "apples" }
{ "_id": 2, "name": "bananas" }
{ "_id": 3, "name": "oranges" }
{ "_id": 4, "name": "avocados" }

Despite the fact that this projection only explicitly included the name field, the query returned the _id field as well!尽管这个投影只显式包含name字段，但查询也返回了_id字段！

This happens because the _id field is a special case: it is always included in every query unless explicitly specified otherwise. 之所以会出现这种情况，是因为_id字段是一种特殊情况：除非另有明确规定，否则它总是包含在每个查询中。That's because _id is a unique identifier for each document, a property that can be very useful when constructing queries. 这是因为_id是每个文档的唯一标识符，这个属性在构造查询时非常有用。The movies collection is a good example of why this property is useful: because remakes and even separate works can sometimes reuse movie titles, you need a unique _id value to refer to any specific movie. movies集合是一个很好的例子，说明了此属性的用途：因为重拍甚至单独的作品有时可以重用电影标题，所以您需要一个唯一的_id值来引用任何特定的电影。_id is the only exception to the mutually exclusive include-exclude behavior in projections: you can explicitly exclude _id even when explicitly including other fields if you do not want _id to be present in returned documents._id是投影中互斥的include-exclude行为的唯一例外：如果不希望返回的文档中存在_id，则即使在显式包含其他字段时，也可以显式排除_id。

// return only the name field
const projection = { _id: 0, name: 1 };
const cursor = collection.find().project(projection);
await cursor.forEach(console.dir);

The projection document specifies a value of 1 for name to indicate that the read operation result should include the name field of each returned document. 投影文档为name指定值1，以指示读取操作结果应包括每个返回文档的name字段。As a result, this projection implicitly excludes the qty and rating fields. 因此，该投影隐式排除了qty和rating字段。Passing this projection to find() with an empty query document and no sort document yields the following results:将此投影传递给带有空查询文档且没有排序文档的find()，将产生以下结果：

{ "name": "apples" }
{ "name": "bananas" }
{ "name": "oranges" }
{ "name": "avocados" }

You can also specify multiple fields to include in your projection. 还可以指定要包含在投影中的多个字段。Note: the order in which you specify the fields in the projection does not alter the order in which they are returned.注意：在投影中指定字段的顺序不会改变它们的返回顺序。

const projection = { _id: 0, rating: 1, name: 1 };
const cursor = collection.find().project(projection);
await cursor.forEach(console.dir);

This example that identifies two fields to include in the projection yields the following results:此示例确定了要包含在投影中的两个字段，并得出以下结果：

  { "name": "apples", "rating": 3 }
  { "name": "bananas", "rating": 1 }
  { "name": "oranges", "rating": 2 }
  { "name": "avocados", "rating": 5 }

For additional projection examples, see the MongoDB Manual page on Project Fields to Return from Query.有关其他投影示例，请参阅MongoDB手册中有关从查询返回的项目字段的页面。