It appears when I try to use simple_form's check_box collection generator.
The problem appears when action_view tries to check type of instance of Mongoid::Relations::Targets::Enumerable.

It can be issue of action_view, if it uses #is_a? method to check types, I think problem will disappear.

Here are direct links:

• https://github.com/mongoid/mongoid/blob/master/lib/mongoid/relations/targets/enumerable.rb
• https://github.com/rails/rails/blob/3-2-13/actionpack/lib/action_view/helpers/form_helper.rb#L1163

Here is code identical to mongoid's Mongoid::Relations::Targets::Enumerable and how action_view checks type with case/when

require "active_support/core_ext/module/delegation"

class T
  include Enumerable

  delegate :===, :is_a?, :kind_of?, to: []
end

t = T.new

p t.is_a?(Array)     # returns true
p t.kind_of?(Array)  # returns true
p t.===(Array)       # returns false
p Array.===(t)       # returns false

# returns "unknown"
case t
when Array
  p "ary"
else
  p "unknown"
end