Collections

MongoDB stores documents in collections. If a collection does not exist, MongoDB creates the collection when you first store data for that collection.

You can also explicitly create a collection with various options, such as setting the maximum size or the documentation validation rules.

Capped Collection

Capped collections have maximum size or document counts that prevent them from growing beyond maximum thresholds. All capped collections must specify a maximum size and may also specify a maximum document count. MongoDB removes older documents if a collection reaches the maximum size limit before it reaches the maximum document count.

To create a capped collection, use the createCollection method and specify 'capped' : true.

const MongoClient = require('mongodb').MongoClient;
const assert = require('assert');

// Connection URL
const url = 'mongodb://localhost:27017';

// Database Name
const dbName = 'myproject';

// Use connect method to connect to the Server
MongoClient.connect(url, function(err, client) {
  assert.equal(null, err);
  console.log("Connected correctly to server");

  const db = client.db(dbName);

  createCapped(db, function() {
    client.close();
  });
});

function createCapped(db, callback) {
  db.createCollection("myCollection", { "capped": true, "size": 100000, "max": 5000},
    function(err, results) {
      console.log("Collection created.");
      callback();
    }
  );
};

Document Validation

Collections with validation compare each inserted or updated document against the criteria specified in the validator option. Depending on the validationLevel and validationAction, MongoDB either returns a warning, or refuses to insert or update the document if it fails to meet the specified criteria.

The following example creates a contacts collection with a validator that specifies that inserted or updated documents should match at least one of three following conditions:

• the phone field is a string
• the email field matches the regular expression
• the status field is either Unknown or Incomplete.

const MongoClient = require('mongodb').MongoClient;
const assert = require('assert');

// Connection URL
const url = 'mongodb://localhost:27017';

// Database Name
const dbName = 'myproject';

// Use connect method to connect to the Server
MongoClient.connect(url, function(err, client) {
  assert.equal(null, err);
  console.log("Connected correctly to server");

  const db = client.db(dbName);

  createValidated(db, function() {
    client.close();
  });
});


function createValidated(db, callback) {
  db.createCollection("contacts", 
	   {
	      'validator': { '$or':
	         [
	            { 'phone': { '$type': "string" } },
	            { 'email': { '$regex': /@mongodb\.com$/ } },
	            { 'status': { '$in': [ "Unknown", "Incomplete" ] } }
	         ]
	      }
	   },
    function(err, results) {
      console.log("Collection created.");
      callback();
    }
  );
};