How to validate if a string is json in a Rails model

Question

I'm building a simple app and want to be able to store json strings in a db. I have a table Interface with a column json, and I want my rails model to validate the value of the string. So something like:

class Interface < ActiveRecord::Base   attr_accessible :name, :json    validates :name,  :presence => true,                     :length   => { :minimum => 3,                                    :maximum => 40 },                     :uniqueness => true    validates :json, :presence => true,                    :type => json #SOMETHING LIKE THIS                    :contains => json #OR THIS     end 

How do I do that?

Answer 1

I suppose you could parse the field in question and see if it throws an error. Here's a simplified example (you might want to drop the double bang for something a bit clearer):

require 'json'  class String   def is_json?     begin       !!JSON.parse(self)     rescue       false     end   end end 

Then you could use this string extension in a custom validator.

validate :json_format  protected    def json_format     errors[:base] << "not in json format" unless json.is_json?   end 

Answer 2

Currently (Rails 3/Rails 4) I would prefer a custom validator. Also see https://gist.github.com/joost/7ee5fbcc40e377369351.

# Put this code in lib/validators/json_validator.rb # Usage in your model: #   validates :json_attribute, presence: true, json: true # # To have a detailed error use something like: #   validates :json_attribute, presence: true, json: {message: :some_i18n_key} # In your yaml use: #   some_i18n_key: "detailed exception message: %{exception_message}" class JsonValidator < ActiveModel::EachValidator    def initialize(options)     options.reverse_merge!(:message => :invalid)     super(options)   end    def validate_each(record, attribute, value)     value = value.strip if value.is_a?(String)     ActiveSupport::JSON.decode(value)   rescue MultiJson::LoadError, TypeError => exception     record.errors.add(attribute, options[:message], exception_message: exception.message)   end  end