Database References数据库引用¶

Background出身背景¶

Using manual references is the practice of including one document’s _id field in another document. 使用手动引用是将一个文档的_id字段包含在另一个文档中的做法。The application can then issue a second query to resolve the referenced fields as needed.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。

Process过程¶

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，使用第二文档中的第一文档的_id字段作为引用，插入两个文档：

original_id = ObjectId()
db.places.insert({
    "_id": original_id,
    "name": "Broadway Center",
    "url": "bc.example.net"
})
db.people.insert({
    "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集合中的places_id字段引用的文档进行第二次查询。

Use¶

For nearly every case where you want to store a relationship between two documents, use manual references. 对于几乎所有要存储两个文档之间关系的情况，都要使用手动引用。The references are simple to create and your application can resolve references as needed.引用易于创建，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. 手动链接的唯一限制是这些引用不传递数据库和集合名称。If you have documents in a single collection that relate to documents in more than one collection, you may need to consider using DBRefs.如果单个集合中的文档与多个集合中的文档相关，则可能需要考虑使用DBRFS。

Background出身背景¶

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

Format总体安排¶

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。

Only some drivers support $db references.只有一些驱动程序支持$db引用。

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

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

Driver Support for DBRefsDBRefs的驱动程序支持¶

Use¶

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