custom_error_messages.markdown

How to customize the humanized :category symbol in the generated error message. Tested with Rails 4.1

class Listing < ActiveRecord::Base    
  validate :premium_category

  private

  def premium_category
    if !user.is_premium && %w(tigers lions rhinos).include?(category))
      errors.add(:category, "not valid for non premium users")
    end
  end
end

config/locales/en.yml

en
 activerecord:
    attributes:
      listing:
        category: "This category is "

This will generate the following error message:

This category is not valid for non premium users