Ruby on Rails: Numerical String Validation

Disclaimer: I don't claim this to be the most effective way to validate a 'numerical string' in rails.
Instead, I wanted to share this to hopefully save people some trouble.
Suppose you have a certain value in your rails model such as a social security number, passport number, etc.
At first thought one might try to save this as an 'integer' datatype. There are a few problems with this though...

1. Instead of validating for length, you'll have to validate by the integer value (must be greater than or less than).
2. Most importantly, anything that begins with a 000 will not save properly because integers in Ruby cannot begin with 0.

• For example, lets say you have a value which you want to save as an integer, like '001234'.
• This won't save as '001234', and it might not be apparent if you weren't aware that Try this in the rails console. If you type in a variable like number = 001234, Ruby will interpret it as 668.
• What is the science behind this? It's all in Ruby's Integer class! Read this StackOverflow post for more information!

3. To get around this, save it as a string! Suppose you have a model field that can only be 12 numbers. An example validation could be as follows:

class ExampleModel < ActiveRecord::Base
  validates :model_field, uniqueness: true, presence: true,
  length: { minimum: 12, maximum: 12, message: 'must be 12 numbers' }
  validate :string_only_contain_numbers    
  
  def string_only_contain_numbers
    if model_field.nil?
      errors.add(:model_field, 'cannot be blank')
    else
      string_numbers = (0..9).to_a.join('').to_s.split('')
      model_field_chars = model_field.split('')
      model_field_chars.each do |num|
        errors.add(:model_field, 'can only contain valid numbers (0-9).') unless string_numbers.include?(num)
      end
    end
  end
end

UPDATE: A more simpler implementation, suggested by a colleague:

def string_only_contain_numbers
  errors.add(:code, 'can only contain numbers 0-9') unless code.scan(/\D/).empty? # will return any non digit character
end

4. In this case, I simply created an array of numbers where the numbers are saved as strings ['0', '1', '2'] and then broke the model_field into an array, and compared the two arrays. If the model_field contains a value that isn't one of the numerical string values, it throws an error. voila! Saving it as a string can also make your other validatiosn simpler because you can validate the length like a string, rather than setting a minimum or maximum value for the number.
   

5. For your testing environment, if you're using FactoryBot (formerly FactoryGirl) or just want to generate a string of random numbers, you can do something like this:

FactoryBot.define do
  # Will generate a string of 12 numerals. 
  sequence :numerical_string do |n|
    ((1..1000).to_a.shuffle.join.to_s + n.to_s)[0, 12]
  end
end

# Without FactoryBot
((1..1000).to_a.shuffle.join.to_s)[0, 12]

Hope this helps someone!