How to validate if a string is json in a Rails model - ruby-on-rails

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

Answer 3

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
end
end
Now you'll be enable to use it anywhere ('controller, model, view,...'), just like this :
puts 'it is json' if JSON.is_json?(something)

Answer 4

I faced another problem using Rails 4.2.4 and PostgreSQL adapter (pg) and custom validator for my json field.
In the following example:
class SomeController < BaseController
def update
#record.json_field = params[:json_field]
end
end
if you pass invalid JSON to
params[:json_field]
it is quietly ignored and "nil" is stored in
#record.json_field
If you use custom validator like
class JsonValidator < ActiveModel::Validator
def validate(record)
begin
JSON.parse(record.json_field)
rescue
errors.add(:json_field, 'invalid json')
end
end
end
you wouldn't see invalid string in
record.json_field
only "nil" value, because rails does type casting before passing your value to validator. In order to overcome this, just use
record.json_field_before_type_cast
in your validator.

Answer 5

If you don't fancy enterprise-style validators or monkey-patching the String class here's a simple solution:
class Model < ApplicationRecord
validate :json_field_format
def parsed_json_field
JSON.parse(json_field)
end
private
def json_field_format
return if json_field.blank?
begin
parsed_json_field
rescue JSON::ParserError => e
errors[:json_field] << "is not valid JSON"
end
end
end

Answer 6

Using JSON parser, pure JSON format validation is possible. ActiveSupport::JSON.decode(value) validates value "123" and 123 to true. That is not correct!
# 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)
if value.is_a?(Hash) || value.is_a?(Array)
value = value.to_json
elsif value.is_a?(String)
value = value.strip
end
JSON.parse(value)
rescue JSON::ParserError, TypeError => exception
record.errors.add(attribute, options[:message], exception_message: exception.message)
end
end

Answer 7

The most simple and elegant way, imo. The top upvoted answers will either return true when passing a string containing integers or floats, or throw an error in this case.
def valid_json?(string)
hash = Oj.load(string)
hash.is_a?(Hash) || hash.is_a?(Array)
rescue Oj::ParseError
false
end