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 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.