How to implement a singleton model

Question

I have a site in rails and want to have site-wide settings. One part of my app can notify the admin by SMS if a specific event happens. This is an example of a feature that

Answer 1

I will put a few remarks to the previous answers:

• no need for a separate field for uniq index, we can put the constraint on the id field. Since we should have id field in a Rails app anyway, it's a good tradeoff
• no need special validations on the model and explicit ID assignment, it's enough to put default value on the column

If we apply these modifications, the solution becomes very easy:

# migration
create_table :settings, id: false do |t|
  t.integer :id, null: false, primary_key: true, default: 1, index: {unique: true}
  t.integer :setting1
  t.integer :setting2
  ...
end

# model
class Settings < ApplicationRecord
  def self.instance
    first_or_create!(...)
  end  
end