MongoDB database reference

MongoDB database reference

In the previous section, we mentioned the MongoDB reference to regulate the data structure document.

There are two types of MongoDB references:

• Manual References
• DBRefs

DBRefs vs manual reference

Consider a scenario where we store different addresses (address, office address, email address, etc.) in different collections (address_home, address_office, address_mailing, etc.).

This way, when we call different addresses, we also need to specify a collection, a document references documents from multiple collections, and we should use DBRefs.

Use DBRefs

In the form of DBRef:

{ $ref : , $id : , $db :  }

The three fields represent the meaning of:

• $ref: Collection name
• $id: The referenced id
• $db: database name, optional parameters

The following example of a user data document uses DBRef, the field address:

{
   "_id":ObjectId("53402597d852426020000002"),
   "address": {
   "$ref": "address_home",
   "$id": ObjectId("534009e4d852427820000002"),
   "$db": "w3cschoolcc"},
   "contact": "987654321",
   "dob": "01-01-1991",
   "name": "Tom Benzamin"
}

address The DBRef field specifies that the referenced address document is the w3cschoolcc database under the address_home collection, with id 534009e4d852427820000002.

In the following code, we look for the user $ref information for the specified id in the collection by specifying the address_home parameter (the collection):

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

The above example returns address_home data from the collection:

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