MongoDB Create Collection

Collection

Collections are like tables in a NoSQL database system like MongoDB. The data records are BSON document stored in collections inside a database. BSON stands for Binary JSON. Collections store documents. Documents are the unit of data stored in collections. Collections can store document which is not same in structure because MongoDB is schema-free DBMS.

{
    “name” : “John”,
    “age” : 22,
    “email” : “example@gmail.com”
}

Example of a collection

A collection name must start with _ (underscore) or letters. A collection name can contain number but not in first space. A collection name can’t contain “$” symbol. The maximum length for a collection is 128 characters.

MongoDB creates collections when we first store the data in the collection.

Create Collection

db.createCollection() method is use to explicitly create a collection. Collections do not need a schema.

db.createCollection() method takes following arguments.

Creating a collection using Javascript

var MongoClient = require('mongodb').MongoClient;
var url = "mongodb://localhost:27017/mydb";
MongoClient.connect(url, function(err, db) {
 if (err) throw err;
 db.createCollection("customers", function(err, res) {
   if (err) throw err;
   console.log("Collection created!");
   db.close();
 });
});

List all collections

You can list all collections in a database with the following command.

1. db.getCollectionNames() – JS Shell
2. db.listCollections() – Node JS
3. show collections –Mongo Shell

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Capped Collection

MongoDB can apply a size limit on a collection. These collections are called a capped collection. Capped collections are implemented using a circular queue and used for high throughput operations. When size limit reaches mongo DB overwrite older documents to make space for new documents without the explicit use of any command.

Capped collections are suitable for storing a high volume of data which need to be refreshed periodically like log data and cache data. Capped collection store document in order of disk storage hence use some restriction. If a document size increases when updating it mongoDB will not update it.

Capped collections cannot be sharded. Documents from capped collection cannot be deleted and must use { emptycapped: nameOfCollection } command to empty the capped collection.

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Creating a capped collection

To create a capped collection db.createCollection() method is used with document option capped set to true and size must be defined in bytes.

db.createCollection( "logdata", { capped: true, size: 1024000 } )
to set a max number of a document we can use the max option also
db.createCollection( "log", { capped: true, size: 100000, max: 2000 } )
To convert a collection to capped collection we can use the below command
db.runCommand({"convertToCapped": "logdata", size: 1024000});
In capped collection there is no default index present.
The capped collection can be used to handle failure scenario. To rollback activities using capped collection because it guarantees the insertion order.
Capped Collection operation in Javascript using node js
var MongoClient = require('mongodb').MongoClient, Server = require('mongodb').Server;
var mongoclient = new MongoClient(new Server(host, port));
var db = mongoclient.db(dbName);
db.open(function(err, db)
{
if (err) throw err;
var capped = db.collection('capped');
var document = {"foo": 1,"bar": 1}
capped.find().count(function(err, count)
{
 if (err) throw err;
 if (count === 0)
 {
  console.log("Creating collection...");
  db.createCollection("capped", {"capped": true,"size": 100000,"max": 5000}, function(err, collection) {
   if (err) throw err;
   console.log("Inserting document...");
   collection.insert(document, function(err, result) {
    if (err) throw err;
   });
  });
} else {
  console.log("Inserting document without creating collection...");
  capped.insert(document, function(err, result) {
   if (err) throw err;
  });
 }
});
});