MongoDB - Database References

As seen in the last chapter of MongoDB relationships, to implement a normapzed database structure in MongoDB, we use the concept of Referenced Relationships also referred to as Manual References in which we manually store the referenced document s id inside other document. However, in cases where a document contains references from different collections, we can use MongoDB DBRefs.

DBRefs vs Manual References

As an example scenario, where we would use DBRefs instead of manual references, consider a database where we are storing different types of addresses (home, office, maipng, etc.) in different collections (address_home, address_office, address_maipng, etc). Now, when a user collection s document references an address, it also needs to specify which collection to look into based on the address type. In such scenarios where a document references documents from many collections, we should use DBRefs.

Using DBRefs

There are three fields in DBRefs −

$ref − This field specifies the collection of the referenced document

$id − This field specifies the _id field of the referenced document

$db − This is an optional field and contains the name of the database in which the referenced document pes

Consider a sample user document having DBRef field address as shown in the code snippet −

{
   "_id":ObjectId("53402597d852426020000002"),
   "address": {
   "$ref": "address_home",
   "$id": ObjectId("534009e4d852427820000002"),
   "$db": "tutorialspoint"},
   "contact": "987654321",
   "dob": "01-01-1991",
   "name": "Tom Benzamin"
}

The address DBRef field here specifies that the referenced address document pes in address_home collection under tutorialspoint database and has an id of 534009e4d852427820000002.

The following code dynamically looks in the collection specified by $ref parameter (address_home in our case) for a document with id as specified by $id parameter in DBRef.

>var user = db.users.findOne({"name":"Tom Benzamin"})
>var dbRef = user.address
>db[dbRef.$ref].findOne({"_id":(dbRef.$id)})

The above code returns the following address document present in address_home collection −

{
   "_id" : ObjectId("534009e4d852427820000002"),
   "building" : "22 A, Indiana Apt",
   "pincode" : 123456,
   "city" : "Los Angeles",
   "state" : "Capfornia"
}