How to get Schema of mongoose database which defined in another model

You can get models defined elsewhere directly with Mongoose:

require('mongoose').model(name_of_model)

To get the schema in your example in albums.js you can do this:

var SongSchema = require('mongoose').model('Song').schema

To get the schema from a registered Mongoose model, you need to access the schema specifically:

var SongSchema = require('mongoose').model('Song').schema;