Change type of field inside mongoDB aggregation and does $lookup utilises index on fields or not?

Question

I am performing joins in mongodb using $lookup, now i am facing a problem here. I have two collections first one to contains users all bookmarks brands and the second one co

Answer 1

You cannot convert the string to a object Id within the pipeline, you'll have to go though each document and convert it manually, using something like (you shouldnt be storing a mix match of types anyway, so it's probably worth updating in the long run):

how to convert string to numerical values in mongodb

as for does $lookup use index, If you look at the stats from this blog you'll see that indexes are used -

http://guyharrison.squarespace.com/blog/2016/7/4/join-performance-in-mongodb-32-using-lookup.html

Answer 2

Try casting your brands to ObjectIds before your population:

user_bookmarked.brands.map((brand) => return mongoose.Types.ObjectId(brand) )

But you really should consider storing them as refs instead, your model should look something like:

const user_bookmarked = new mongoose.Schema({
    ...
    brands: [{type: mongoose.Schema.Types.ObjectId, ref: 'Brands'}],
    ...
    })

This way they will be ObjectIds from the start.!

Regarding the second question this post explains it I think: join-performance-in-mongodb-32-using-lookup