MongoDB merge two same schema collection - node.js

Question

I am using mongoDB with node express and I have two collection in my db having name "user" and "user_archive". I want to merge both collection data in a query and want to perform pagination.
For single collection pagination I am using below code:
user.find(condition)
.select()
.limit(perPage)
.skip(perPage * page)
.sort(sortObject)
.exec(function(err, data)
{
callback(err,data);
});
I want to perform pagination by merging both user and user_archive collection data.
user collection
{"name":A}
{"name": B}
user_archive collection
{"name":C}
{"name":D}
I need to merge both collection like below and then want to perform pagination:
{"name":A}
{"name":B}
{"name":C}
{"name":D}
Could anyone please suggest how can I do that in my code.

Answer 1

You can use aggregation piplines.
For example
User collection
{ "_id": "1", "email": "test1#gmail.com", "fullName": "Jon1"},
{ "_id": "2", "email": "test2#gmail.com", "fullName": "Jon2"},
{ "_id": "3", "email": "test3#gmail.com", "fullName": "Jon3"}
Post Collection
{ "_id": "11", "owner": "1", "content": "text1"},
{ "_id": "12", "owner": "2", "content": "text2"},
{ "_id": "13", "owner": "3", "content": "text3"},
.
async function getData (options) {
let query = [];
// merge posts and users
query.push(
{ $lookup: { from: 'posts', localField: '_id', foreignField: 'owner', as: 'posts' } }
);
// pagination
if (options.page && options.size) {
let skip = options.size * (options.page - 1);
query.push(
{ $skip: skip },
{ $limit: options.size }
);
}
return await db.posts.aggregate(query);
}
The result from this function will be
{ "_id": "11", "owner": { "_id": "1", "email": "test1#gmail.com", "fullName": "Jon1" }, "content": "text1"},
{ "_id": "12", "owner": { "_id": "2", "email": "test2#gmail.com", "fullName": "Jon2" }, "content": "text2"},
{ "_id": "13", "owner": { "_id": "3", "email": "test3#gmail.com", "fullName": "Jon3" }, "content": "text3"}
For example if options = { page: 1, size: 2 }, result will be
{ "_id": "11", "owner": { "_id": "1", "email": "test1#gmail.com", "fullName": "Jon1" }, "content": "text1"},
{ "_id": "12", "owner": { "_id": "2", "email": "test2#gmail.com", "fullName": "Jon2" }, "content": "text2"},