Mongoose.model vs Connection.model vs Model.model

I am bit confused with usage of models in mongoosejs

Models can be created using mongoose in these ways

Using Mongoose 

var mongoose = require('mongoose');
var Actor = mongoose.model('Actor', new Schema({ name: String }));

Using Connection 

var mongoose = require('mongoose');
var db = mongoose.createConnection(..);
db.model('Venue', new Schema(..));
var Ticket = db.model('Ticket', new Schema(..));
var Venue = db.model('Venue');

Using existing Model instance 

var doc = new Tank;
doc.model('User').findById(id, callback);

Now what is the difference between model returned by Mongoose.model , Connection.model and Model.model. and when to use what , what is the recommended way to create/fetch model ?

  1. mongoose.model ties the defined model to the default connection that was created by calling mongoose.connect.
  2. db.model ties the model to the connection that was created by calling var db = mongoose.createConnection.
  3. doc.model looks up another model by name using the connection that doc's model is tied to.

All three can be sensibly used in the same program; which one to use just depends on the situation.

ok here is what I found

Important! If you opened a separate connection using mongoose.createConnection() but attempt to access the model through mongoose.model('ModelName') it will not work as expected since it is not hooked up to an active db connection. In this case access your model through the connection you created:

var conn = mongoose.createConnection('your connection string');
var MyModel = conn.model('ModelName', schema);
var m = new MyModel;
m.save() // works

vs

var conn = mongoose.createConnection('your connection string');
var MyModel = mongoose.model('ModelName', schema);
var m = new MyModel;
m.save() // does not work b/c the default connection object was never connected