It appears that the recent additions of adding scopes (which is great!) doesn't support the API of passing unmodified ActionController::Parameters.

Assuming:

class Person < ActiveRecord::Base
    scope :domestic, -> { where(country: 'US') }
    def self.ransackable_scopes(auth_object=nil); [:domestic]; end
end

Here's a few examples:

Person.search(domestic: true) # scopes just perfectly

Form helper using a hidden input

<%= f.hidden_field :domestic %>

If the input value is set to true, it is deserialized from the request as "true".

Calling Person.search(params[:q]) will raise ArgumentError: wrong number of arguments (1 for 0) as Ransack tries to pass "true" to the scope.

If the hidden value is set to "", Ransack skips applying the scope.

For now, we are able to manually deserialize these parameters in to the correct values, but just wanted to give you a heads up.

I should be able to look in to ransack's code some more this weekend.