转自：https://docs.mongodb.com/manual/reference/database-references/
MongoDB does not support joins. In MongoDB some data is denormalized, or stored with related data in documents to remove the need for joins. However, in some cases it makes sense to store related information in separate documents, typically in different collections or databases.
MongoDB applications use one of two methods for relating documents:
- Manual references where you save the
_idfield of one document in another document as a reference. Then your application can run a second query to return the related data. These references are simple and sufficient for most use cases. - DBRefs are references from one document to another using the value of the first document’s
_idfield, collection name, and, optionally, its database name. By including these names, DBRefs allow documents located in multiple collections to be more easily linked with documents from a single collection.
To resolve DBRefs, your application must perform additional queries to return the referenced documents. Many drivers have helper methods that form the query for the DBRef automatically. The drivers do not automatically resolve DBRefs into documents.
DBRefs provide a common format and type to represent relationships among documents. The DBRef format also provides common semantics for representing links between documents if your database must interact with multiple frameworks and tools.
Unless you have a compelling reason to use DBRefs, use manual references instead.