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MongoDB database references

In the previous section we mentioned the relationship MongoDB MongoDB reference to standardize the data structure of the document.

MongoDB has two references:

  • Reference manual (Manual References)
  • DBRefs

DBRefs vs reference manual

Consider such a scenario, we have a different set (address_home, address_office, address_mailing, etc.) stored in a different address (address, office address, mailing address, etc.).

Thus, when we call different address, you need to specify a collection, a collection of documents from multiple referenced documents, we should use DBRefs.

Use DBRefs

DBRef forms: 

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

Significance of three fields represented by:

  • $ Ref: collection name
  • $ Id: reference id
  • $ Db: database name, optional parameters

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

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

address DBRef field specifies the address referenced documents are under address_home collection w3cschoolcc database, id is 534009e4d852427820000002.

The following code, we have to find the collection at the specified address information user id specified by $ ref parameter (address_home set): 

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

The above examples returned address_home collection address data: 

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