test_params.rb

require 'bundler/setup'
require 'active_support/all'
require 'rack'
require 'rack/test'
require 'action_controller/metal/strong_parameters'
require 'stronger_parameters'
require 'byebug'

input = {
  order: {
    customers_attributes: {
      "0" => {
        'first_name'     => 'Steve',
        'last_name'      => 'Lastnameson',
        'email'          => '[email protected]',
      },
      "1" => {
        'first_name'     => 'Steve',
        'last_name'      => 'Lastnameson',
        'email'          => '[email protected]',
      }
    }
  }
}
params = ActionController::Parameters.new(input)
params.require_hash(:order).permit_types(
  customers_attributes: Array
)
# raises UnpermittedParameterType because customer_attributes is technically a Hash, not an Array