The Object #=== method is defined as "case equality", being the equality that you'd like to enforce in case statements. In mongoid this is now class-equality. This means you can build a case statement in mongoid around classes, but not regular mongoid document instances.

Take the example below:

class A
  include Mongoid::Document
end

a = A.new
b = A.new

puts a == a # Returns true
puts a == b # Returns false
puts a === b # Returns true as mongoid === only checks class equality

case a
  when b
    puts "Printed because A and B are the same class... This doesn't seem intuitive"
  when a
    puts "This should be printed, but isn't."
end

case a.id
  when b.id
    puts "Not printed as they don't match"
  when a.id
   puts "This is printed as the id === id uses the Ruby Object rules"
end

Having a specific class/subclass match does seem useful if one of the objects is a class – however, if they are both regular instances it would make more sense to use equals.

Current workaround is to use the id insstead. e.g. "case a.id...".