Database References数据库参考

Background背景

A manual reference is the practice of including one document's _id field in another document. The application can then issue a second query to resolve the referenced fields as needed.手动参考是将一个文档的_id字段包含在另一个文档中的做法。然后，应用程序可以根据需要发出第二个查询来解析引用的字段。

Create a Manual Reference in the MongoDB Atlas UI在MongoDB Atlas UI中创建手册参考

To create a manual reference in the MongoDB Atlas UI, follow these steps:要在MongoDB Atlas UI中创建手动参考，请执行以下步骤：

Create a Manual Reference in the Terminal在终端中创建手动参考

Consider the following operation to insert two documents, using the _id field of the first document as a reference in the second document:考虑以下操作来插入两个文档，使用第一个文档的_id字段作为第二个文档中的引用：

original_id = ObjectId()

db.places.insertOne({
    "_id": original_id,
    "name": "Broadway Center",
    "url": "bc.example.net"
})

db.people.insertOne({
    "name": "Erin",
    "places_id": original_id,
    "url":  "bc.example.net/Erin"
})

Then, when a query returns the document from the people collection you can, if needed, make a second query for the document referenced by the places_id field in the places collection.然后，当查询从people集合返回文档时，如果需要，您可以对places集合中places_id字段引用的文档进行第二次查询。

Use使用

For nearly every case where you want to store a relationship between two documents, use manual references. The references are simple to create and your application can resolve references as needed.对于几乎所有想要存储两个文档之间关系的情况，请使用手动引用。创建引用很简单，应用程序可以根据需要解析引用。

The only limitation of manual linking is that these references do not convey the database and collection names. If you have documents in a single collection that relate to documents in more than one collection, you may need to consider using DBRefs.手动链接的唯一限制是这些引用不传达数据库和集合名称。如果单个集合中的文档与多个集合中的文件相关，则可能需要考虑使用DBRefs。

Background背景

DBRefs are a convention for representing a document, rather than a specific reference type. They include the name of the collection, and in some cases the database name, in addition to the value from the _id field.DBRef是一种表示文档的约定，而不是特定的引用类型。除了_id字段中的值外，它们还包括集合的名称，在某些情况下还包括数据库名称。

Optionally, DBRefs can include any number of other fields. Extra field names must follow any rules for field names imposed by the server version.DBRef可以视需要包含任意数量的其他字段。额外的字段名必须遵循服务器版本强加的字段名规则。

Format格式

DBRefs have the following fields:DBRef具有以下字段：

$ref

The $ref field holds the name of the collection where the referenced document resides.$ref字段保存引用文档所在的集合的名称。

$id

The $id field contains the value of the _id field in the referenced document.$id字段包含引用文档中_id字段的值。

$db

Optional.

Contains the name of the database where the referenced document resides.包含引用文档所在的数据库的名称。

{ "$ref" : <value>, "$id" : <value>, "$db" : <value> }

{
  "_id" : ObjectId("5126bbf64aed4daf9e2ab771"),
  // .. application fields
  "creator" : {
                  "$ref" : "creators",
                  "$id" : ObjectId("5126bc054aed4daf9e2ab772"),
                  "$db" : "users",
                  "extraField" : "anything"
               }
}

Driver Support for DBRefsDBRefs的驱动程序支持

Use使用

In most cases you should use the manual reference method for connecting two or more related documents. However, if you need to reference documents from multiple collections, consider using DBRefs.在大多数情况下，您应该使用手动引用方法来连接两个或多个相关文档。但是，如果需要引用多个集合中的文档，请考虑使用DBRefs。