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.
Starting in version 3.2, you can use
$lookup to perform a left outer join to an unsharded collection in the same database.
For more information and examples, see
This page outlines alternative procedures that predate the
$lookup pipeline stage.
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.
|||Some community supported drivers may have alternate behavior and may resolve a DBRef into a document automatically.|
Using manual references 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.
Consider the following operation to insert two documents, using the
_id field of the first document as a reference in the second document:
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
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.
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.
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
DBRefs have the following fields:
$reffield holds the name of the collection where the referenced document resides.
$idfield contains the value of the
_idfield in the referenced document.
Contains the name of the database where the referenced document resides.
Only some drivers support
DBRef documents resemble the following document:
Consider a document from a collection that stored a DBRef in a
The DBRef in this example points to a document in the
creators collection of the
users database that has
ObjectId("5126bc054aed4daf9e2ab772") in its
The order of fields in the DBRef matters, and you must use the above sequence when using a DBRef.
|C||Not Supported||You can traverse references manually.|
|C++||Not Supported||You can traverse references manually.|
|C#||Supported||Please see the C# driver page for more information.|
|Haskell||Not Supported||You can traverse references manually.|
|Java||Supported||Please see the Java driver page for more information.|
|Node.js||Supported||Please see the Node.js driver page for more information.|
|Perl||Supported||Please see the Perl driver page for more information.|
|PHP||Not Supported||You can traverse references manually.|
|Python||Supported||Please see the PyMongo driver page for more information.|
|Ruby||Supported||Please see the Ruby driver page for more information.|
|Scala||Not Supported||You can traverse references manually.|
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.