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;

For others not as familiar with the deeper aspects of how Mongoose works, the existing answers can be confusing.

Here's a generalized implementation example of importing a schema from another file that is accessible to a wider audience coming from a more general context.

const modelSchema = require('./model.js').model('Model').schema

Here's a modified version for the specific case in the question (this would be used inside albums.js).

const SongSchema = require('./songs.js').model('Song').schema

From this, I can see that you first access and require the file how one would normally go about requiring a model, except in this case you now specifically access the schema of that model.

Other answers require mongoose within the variable declaration and that goes against the commonly found example of requiring mongoose before through declaring a variable such as const mongoose = require('mongoose'); and then using mongoose like that. Such a use case was not accessible knowledge-wise to me.

Alternative option

You can require just the model like you normally would and then refer to the schema through the Model's schema property.

const mongoose = require('mongoose');

// bring in Song model
const Song = require('./songs.js');

const AlbumSchema = new Schema({
    // access built in schema property of a model
    songs: [Song.schema]
});

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

The above line of code in your albums.js will surely work.