I've noted the issue on the original PR which integrated the code here: https://github.com/mongodb/mongo-ruby-driver/pull/619

Issue seems that I can provide a filter for a .find() method but the parallel_scan returns cursors that do not apply the given filter.

An example, reproduced for you here:

mongo[:users].find(firstName: 'Jerry').parallel_scan(8).each do |cursor|
cursor.each do |u|
u[:firstName] # This shows firstName of ALL users (i.e. no filter applied)
end
end