How to search for a document via ref to another model?

Question

MongoDB

I have a User model that has an email field that is important for my task. I
also have an Employee model that has a user field: { ref: "User" }
the question is: how can I find an employee by email?
I did it through employee.find({}).populate() and from this result I go through each one and look for email. The
problem with this approach is that if there are a very large number of records in the database, then these operations will take a lot of time