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Database References

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  • Manual References
  • DBRefs

For many use cases in MongoDB, the denormalized data model where related data is stored within a single document is optimal. However, in some cases, it makes sense to store related information in separate documents, typically in different collections or databases.

Important

You can use the $lookup pipeline stage to perform a left outer join to an unsharded collection in the same database.

You can also use the $graphLookup pipeline stage to join an unsharded collection to perform recursive search.

This page outlines alternative procedures that predate the $lookup and $graphLookup pipeline stages.

MongoDB applications use one of two methods to relate documents:

  • Manual references save the _id field of one document in another document as a reference. Your application runs 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 _id field, collection name, and, optionally, its database name, as well as any other fields. DBRefs allow you to more easily reference documents stored in multiple collections or databases.

To resolve DBRefs, your application must perform additional queries to return the referenced documents. Some MongoDB drivers provide helper methods to enable DBRefs to be resolved into documents, but it doesn't happen automatically.

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.

Manual References

Background

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.

Process

Consider the following operation to insert two documents, using the _id field of the first document as a reference in the second document:

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.

Use

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

Background

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.

Optionally, DBRefs can include any number of other fields. Extra field names must follow any rules for field names imposed by the server version.

Format

DBRefs have the following fields:

$ref

The $ref field holds the name of the collection where the referenced document resides.

$id

The $id field contains the value of the _id field in the referenced document.

$db

Optional.

Contains the name of the database where the referenced document resides.

Example

DBRef documents resemble the following document:

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

Consider a document from a collection that stored a DBRef in a creator field:

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

The DBRef in this example points to a document in the creators collection of the users database that has ObjectId("5126bc054aed4daf9e2ab772") in its _id field. It also contains an optional field.

Note

The order of fields in the DBRef matters, and you must use the above sequence when using a DBRef.

Driver Support for DBRefs

Driver
DBRef Support
Notes
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.
Go
Not Supported
You can traverse references manually.
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.

Use

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.

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