Virtus.md

# gemfile
gem 'virtus'

# view
render 'forms/errors', form: @form
form_for @form, url: my_path do |f|
  f.input
  f.submit
end

# terms_controller.rb
def new
  @form = build_terms_form
end

def create
  @form = build_terms_form
  if @form.valid?
    # some code
  else
    # some other code
  end
end

def build_terms_form
  return TermsForm.new unless params[:terms_form].present?
  TermsForm.new(params[:terms_form])
end

# app/forms/terms_form.rb
class TermsForm
  include ActiveModel::Model
  include Virtus.model
  
  attribute :corporate_name, String
  attribute :legal_representer, Boolean
  
  validates_inclusion_of :legal_representer, in: [true], message: 'must be accepted'
  validate :valid_corporate_name?
  
  def valid_corporate_name?
    errors.add(:corporate_name, 'is not a real name') unless corporate_name.size < 2
  end