Reference to schema (not a particular document) in mongoose - mongodb

Question

Please, note that this is not a duplicate of this, nor this, nor this, since what I need is not a reference to a document from another collection, but a reference to the collection itself.
I'm using mongoose-schema-extend to create a hierarchic structure for contents.
Let's say I have this:
/**
* Base class for content
*/
var ContentSchema = new Schema({
URI: {type: String, trim: true, unique: true, required: true },
auth: {type: [Schema.Types.ObjectId], ref: 'User'},
timestamps: {
creation: {type: Date, default: Date.now},
lastModified: {type: Date, default: Date.now}
}
}, {collection: 'content'}); // The plural form of content is content
/**
* Pages are a content containing a body and a title
*/
var PageSchema = ContentSchema.extend({
title: {type: String, trim: true, unique: true, required: true },
format: {type: String, trim: true, required: true, validate: /^(md|html)$/, default: 'html' },
body: {type: String, trim: true, required: true}
});
/**
* Articles are pages with a reference to its author and a list of tags
* articles may have a summary
*/
var ArticleSchema = PageSchema.extend({
author: { type: Schema.Types.ObjectId, ref: 'User', required: true },
summary: { type: String },
tags: { type: [String] }
});
Now, I want to create another schema, which is a subtype of content, but which represents a set of contents, like so:
/**
* Content sets are groups of content intended to be displayed by views
*/
var ContentSetSchema = ContentSchema.extend({
name: {type: String, trim: true, unique: true, required: true },
description: {type: String },
content: [{
source: { type: [OTHER_SCHEMA] }, // <- HERE
filter: {type: String, trim: true },
order: {type: String, trim: true }
}]
})
So, the content attribute of this schema should be a reference to any of the other schemas.
Is it possible?

Answer 1

The best I came up whit, was using a string, a discriminator key, and a validator:
var ContentSchema = new Schema({
// ...
}, {collection: 'content', discriminatorKey : '_type'});
var ContentSetSchema = ContentSchema.extend({
// ...
content: [{
source: { type: [String], validate: doesItExist }
}]
});
function doesItExist(type, result) {
ContentModel.distinct('_type').exec()
.then((contentTypes) =>
respond(contentTypes.some((validType) => validType === type)));
}
But with this solution (good enough an this moment) I can only create ContentSets for types of content that have already some register in the database.