Avo Rails custom validation

gist.rb

class Post < ApplicationRecord
  validate :url_difference

  def url_difference
    # try and find a post with the same URL
    possible_record = Post.where(url: url).first

    # If that exists add to errors
    if possible_record.present?
      errors.add :url, "A post with that URL exists."
    end
  end
end