has_nanoid.rb

# Usage:
#
#   class Post < ApplicationRecord
#     include HasNanoid
#     has_nanoid
#   end
#
module HasNanoid
  extend ActiveSupport::Concern

  ALPHABET = Random::Formatter::ALPHANUMERIC + %w[- _]

  included do
    before_create :generate_nanoid

    validates :nanoid, uniqueness: true, allow_nil: true

    def to_param
      nanoid
    end
  end

  class_methods do
    def has_nanoid(prefix: nil, size: 8)
      @nanoid_prefix = prefix || name.downcase.first(2)
      @nanoid_size = size
    end

    attr_reader :nanoid_prefix, :nanoid_size
  end

  private

  def generate_nanoid
    prefix = self.class.nanoid_prefix
    size = self.class.nanoid_size

    loop do
      id = SecureRandom.alphanumeric(size, chars: ALPHABET)
      self.nanoid = "#{prefix}_#{id}"
      break unless self.class.exists?(nanoid: nanoid)
    end
  end
end