Custom Validator for Max Length based on database adapter

DbMaxLengthValidator.rb

module DbMaxLengthValidatorConcern
  extend ActiveSupport::Concern

  included do
    cattr_accessor :column_varchar_max_lengths
    self.column_varchar_max_lengths = {}

  end

  module ClassMethods
    def load_schema!
      super
      setup_column_varchar_max_lengths
    end

    private
    def setup_column_varchar_max_lengths
      columns.each do |c|
        sql_meta = c.sql_type_metadata
        # only handle varchars and text for now
        next unless ["varchar", "text"].any?{|type| sql_meta.sql_type.match(type) }
        column_varchar_max_lengths[c.name.to_sym] = sql_meta.limit - 1
      end      
    end
  end
  
  class DbMaxLengthValidator < ActiveModel::EachValidator
    def validate_each(record, attribute, value)
      max_length = record.class.column_varchar_max_lengths[attribute]
      return unless value.present? && max_length.present?
      unless value.length < max_length
        record.errors.add attribute, (options[:message] || _("is too long (maximum is #{max_length} characters)"))
      end
    end     
  end
end

message.rb

# example class that has #sms_body string attribute
class Message < ActiveRecord::Base
  include DbMaxLengthValidator
  
  validates :sms_body, db_max_length: true
end