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// Relationships in MongoDB represent how various documents are logically related to each other. Relationships can be modeled via Embedded and Referenced approaches. Such relationships can be either 1:1, 1:N, N:1 or N:N.

//Embedded Relationships
{
   "_id":ObjectId("52ffc33cd85242f436000001"),
   "name": "Tom Benzamin",
   "address": [
      {
         "building": "22 A, Indiana Apt",
         "pincode": 123456,
         "city": "Los Angeles",
         "state": "California"
      },
      {
         "building": "170 A, Acropolis Apt",
         "pincode": 456789,
         "city": "Chicago",
         "state": "Illinois"
      }
   ]
} 
//This approach maintains all the related data in a single document, which makes it easy to retrieve and maintain. The whole document can be retrieved in a single query such as
db.users.findOne({"name":"Tom Benzamin"},{"address":1})

//Referenced Relationships
{
   "_id":ObjectId("52ffc33cd85242f436000001"),
   "contact": "987654321",
   "dob": "01-01-1991",
   "name": "Tom Benzamin",
   "address_ids": [
      ObjectId("52ffc4a5d85242602e000000"),
      ObjectId("52ffc4a5d85242602e000001")
   ]
}
//As shown above, the user document contains the array field address_ids which contains ObjectIds of corresponding addresses. Using these ObjectIds, we can query the address documents and get address details from there. With this approach, we will need two queries: first to fetch the address_ids fields from user document and second to fetch these addresses from address collection.
var result = db.users.findOne({"name":"Tom Benzamin"},{"address_ids":1})
var addresses = db.address.find({"_id":{"$in":result["address_ids"]}})

// 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, mailing, etc.) in different collections (address_home, address_office, address_mailing, 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 lies

{
   "_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 lies in address_home collection under tutorialspoint database and has an id of 534009e4d852427820000002.

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