How do we reference to a collection in MongoDB?
I wanted to know, how can I add a reference to another collection?


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.

It's possible through Modeling Referenced Relationships and DBRefs.

Here i am going to discuss about Modeling Referenced Relationships. 

Let us consider the case of storing addresses for customers. So, one customer can have multiple addresses making this a 1:N relationship.

Following is the sample document structure of customer document

{
"_id":ObjectId("52ffc33cd85242f436000001"),
   "name": "Gokulnath Kumar",
   "gender": "Male",
   "city": "Doha",
   "country": "Qatar",
   "Phone": "777 95057",
   "address": "100A, Doha,Qatar"
})


Following is the sample document structure of address document −

{
"_id":ObjectId("52ffc4a5d85242602e000000"),
   "address": "100A, Doha,Qatar",
   "office address": " 200B, Doha Qatar",
   "mailing address": "abc@gmail.com"
} 


Let's first i am going to write down here Modeling Embedded Relationships, for your better understanding.Even as @JJussi has already told that we can write down all documents information in one collection through modeling Embedded Relationship.


  Note: The drawback is that if the embedded document keeps on growing too much in size, it can impact the read/write performance.


Modeling Embedded Relationships

Data in MongoDB has a flexible schema. Collections do not enforce document structure. Decisions that affect how you model data can affect application performance and database capacity.

This document describes a data model that uses embedded documents to describe relationships between connected data.

If your application frequently retrieves the address data with the name information, then your application needs to issue multiple queries to resolve the references. A more optimal schema would be to embed the address data entities in the customer data, as in the following document:

{
"_id":ObjectId("52ffc33cd85242f436000001"),
    "name": "Gokulnath Kumar",
   "gender": "Male",
   "city": "Doha",
   "country": "Qatar",
   "Phone": "777 95057",
   "address": [
{
"address": "100A, Doha,Qatar",
   "office address": " 200B, Doha Qatar",
   "mailing address": "abc@gmail.com"
}]}


Then Modeling Referenced Relationships

This is the approach of designing normalized relationship. In this approach, both the customer and address documents will be maintained separately but the customer document will contain a field that will reference the address document's id field.

{
   "_id":ObjectId("52ffc33cd85242f436000001"),
   "name": "Gokulnath Kumar",
   "gender": "Male",
   "city": "Doha",
   "country": "Qatar",
   "Phone": "777 95057",
   "address_ids": [
      ObjectId("52ffc4a5d85242602e000000"),
      ObjectId("52ffc4a5d85242602e000001")
   ]
}


As shown above, the customer 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 customer document and second to fetch these addresses from address collection.

&gt;var result = db.customer.findOne({"name":"Gokulnath Kumar"},{"address_ids":1})
&gt;var addresses = db.address.find({"_id":{"$in":result["address_ids"]}})


For further your ref Here, Here and Here