Database Reference
In-Depth Information
To obtain the information itself, you combine the
findOne
function with some dot notation:
> db.publisherscollection.findOne( { _id : book.Publisher } )
{
"_id" : "Apress",
"Type" : "Technical Publisher",
"Category" : [
"IT",
"Software",
"Programming"
]
}
As this example illustrates, referencing data manually is straightforward and doesn't require much brainwork.
Here, the
_id
in the documents placed in the
users
collection has been manually set and has not been generated by
MongoDB (otherwise, the
_id
would be an object ID).
Referencing Data with DBRef
The DBRef standard provides a more formal specification for referencing data between documents. The main reason
for using DBRef over a manual reference is that the collection can change from one document to the next. So, if your
referenced collection will always be the same, the referencing data manually (as just described) is fine.
With DBRef, the database reference is stored as a standard embedded (JSON/BSON) object. Having a standard
way to represent references means that drivers and data frameworks can add helper methods that manipulate the
references in standard ways.
The syntax for adding a DBRef reference value looks like this:
{ $ref :
<collectionname>
, $id :
<id value>
[, $db :
<database name>
] }
Here,
<collectionname>
represents the name of the collection referenced (for example,
publisherscollection
);
<id value>
represents the value of the
_id
field for the object you are referencing; and the optional
$db
allows you to
reference documents that are placed in other databases.
Let's look at another example using DBRef from scratch. Begin by emptying your two collections and adding a
new document:
> db.publisherscollection.drop()
true
> db.media.drop()
true
> apress = ( { "Type" : "Technical Publisher", "Category" :
["IT","Software","Programming"] } )
{
"Type" : "Technical Publisher",
"Category" : [
"IT",
"Software",
"Programming"
]
}
> db.publisherscollection.save(apress)
