In the "when auto protecting ids and types is false" of field_spec.rb, protect_sensitive_fields is used dynamically.. however, it's not working dynamically for me:

1.9.3p194 :005 >   Mongoid.protect_sensitive_fields = false
 => false
1.9.3p194 :006 > MyModel.new(MyModel.first.serializable_hash)
  DEBUG - WARNING: Can't mass-assign protected attributes: _id

Note that it does work if I set the config option in mongoid.yml globally

My setup, from Gemfile.lock:

    mongoid (3.0.9)
      activemodel (~> 3.1)
      moped (~> 1.1)
      origin (~> 1.0)
      tzinfo (~> 0.3.22)

Using mongoid with padrino (only non default option is

: true

in my mongoid.yml)