ruby
require 'mongoid'

class Foo
  include Mongoid::Document

  field :x
  index(x: -1)
  index(x: 1)
end

Foo.index_specifications.each do |spec|
  p spec
end

This only yields a single index specification:

[#<Mongoid::Indexable::Specification:0x007fdb8f45c4b0 @klass=Foo, @key={:x=>-1}, @options={}>] 

This is definitely a change from 3.x, but I wonder whether it's by design? I am not clear whether having an index in both directions actually does anything in MongoDB, so if it's not a regression it should at least be documented.