Aggregate results from multiple Mongoose models - node.js

Question

There are several models that are considerably different and belong to different collections, yet they have common fields that will be used when query results are aggregated together.
const BlogPost = mongoose.model('BlogPost', new mongoose.Schema({
title: String,
body: String,
promoteAtMainPage: {
type: Boolean,
default: false
},
timestamp: Date,
active: Boolean,
/ * the rest are different */
});
const Article = mongoose.model('Article', new mongoose.Schema({
title: String,
body: String,
promoteAtMainPage: {
type: Boolean,
default: false
},
timestamp: Date,
active: Boolean,
/ * the rest are different */
});
Only common fields (title, body, timestamp) are used from the result.
Additionally, if promoteAtMainPage is missing in document, this should be treated differently in these models (it defaults to true in one case and false in another).
Currently this is done with result processing:
let blogPosts = await BlogPost.aggregate([{ $match { active: true } }, { $sort: { timestamp: -1} }, { $limit: 100 } }]);
for (let blogPost of blogPosts)
blogPost.promoteAtMainPage = ('promoteAtMainPage' in blogPost)
? blogPost.promoteAtMainPage
: false;
let articles = await Article.aggregate([{ $match { active: true } }, { $sort: { timestamp: -1} }, { $limit: 100 } }]);
for (let article of articles)
article .promoteAtMainPage = ('promoteAtMainPage' in article )
? article .promoteAtMainPage
: true;
let mainPagePosts = [...blogPosts, ...articles]
.filter(post => post.promoteAtMainPage)
.sort((a, b) => b.timestamp - a.timestamp)
.slice(0, 100);
This results in requesting 200 documents instead of 100 and doing extra sort.
Is this possible in this case to aggregate the results from different collections by means of Mongoose or Mongodb only?
Can the case with missing promoteAtMainPage field be handled by Mongodb as well, or the only reasonable way to handle this is to apply a migration to all existing documents and add default promoteAtMainPage value?

Answer 1

Have a look at $mergeObjects from the MongoDB documentation. It allows you to combine fields from multiple collections and return a new temporary collection with documents combined from the original collections.
https://docs.mongodb.com/manual/reference/operator/aggregation/mergeObjects/
You might be able to use that to combine your BlogPost and Article collections.