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

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  endend

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

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

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)  endend

The best way is to add a method to the JSON module !

Put this in your config/application.rb :

module JSON  def self.is_json?(foo)    begin      return false unless foo.is_a?(String)      JSON.parse(foo).all?    rescue JSON::ParserError      false    end   endend

Now you'll be enable to use it anywhere ('controller, model, view,...'), just like this :

puts 'it is json' if JSON.is_json?(something)