class PeopleController < ActionController::Base
  # Using "Person.create(params[:person])" would raise an
  # ActiveModel::ForbiddenAttributes exception because it'd
  # be using mass assignment without an explicit permit step.
  # This is the recommended form:
  def create
    Person.create(person_params)
  end

  # This will pass with flying colors as long as there's a person key in the
  # parameters, otherwise it'll raise an ActionController::MissingParameter
  # exception, which will get caught by ActionController::Base and turned
  # into a 400 Bad Request reply.
  def update
    redirect_to current_account.people.find(params[:id]).tap { |person|
      person.update!(person_params)
    }
  end

  private
    # Using a private method to encapsulate the permissible parameters is
    # just a good pattern since you'll be able to reuse the same permit
    # list between create and update. Also, you can specialize this method
    # with per-user checking of permissible attributes.
    def person_params
      params.require(:person).permit(:name, :age)
    end
end

class Person
  has_many :pets
  accepts_nested_attributes_for :pets
end

class PeopleController < ActionController::Base
  def create
    Person.create(person_params)
  end

  ...

  private

    def person_params
      # It's mandatory to specify the nested attributes that should be whitelisted.
      # If you use `permit` with just the key that points to the nested attributes hash,
      # it will return an empty hash.
      params.require(:person).permit(:name, :age, pets_attributes: [ :name, :category ])
    end
end