ActiveModel Validator that checks whether the string value of an attribute is a valid ActiveSupport::TimeZone

time_zone_validator.rb

class TimeZoneValidator < ActiveModel::EachValidator
  def validate_each(record, attribute, value)
    unless ActiveSupport::TimeZone[value]
      record.errors[attribute] << (options[:message] || 'is not a valid time zone!')
    end
  end
end

# Usage with rails:
# 
# Put the validator class into +app/validators/time_zone_validators.rb+ and it will be loaded automatically.
# Create a model with a +time_zone+ attribute: ./bin/rails g model user time_zone:string
#
#   class User < ActiveRecord::Base
#     validates :time_zone, presence: true, time_zone: true
#   end
#
# Usage just with ActiveModel:
#
#   class User
#     include ActiveModel::Validations
#     validates :time_zone, presence: true, time_zone:true
#   end