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

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

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 3

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 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

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

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