json type validator of ActiveRecord.

json_type_validator.rb

# Put this code in lib/validators/json_type_validator.rb
# Usage in your model:
#   validates :json_attribute, json_type: true
#
# To have a detailed error use something like:
#   validates :json_attribute, presence: true, json_type: {message: :some_i18n_key}
# In your yaml use:
#   some_i18n_key: "detailed exception message: %{exception_message}"
#
# @see https://gist.github.com/joost/7ee5fbcc40e377369351
class JsonTypeValidator < ActiveModel::EachValidator

  def initialize(options)
    options.reverse_merge!(:message => :invalid)
    super(options)
  end

  def validate_each(record, attribute, value)
    value = record.try(:"#{attribute}_before_type_cast")
    value = value.strip if value.is_a?(String)
    return true if value.blank?
    if value.is_a? String
      ActiveSupport::JSON.decode(value)
    else
      true
    end
  rescue JSON::ParserError => exception
    record.errors.add(attribute, options[:message], exception_message: exception.message)
  end
end