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

Answer 2

I assume to use inheriting column type with uniq constraint.

# miragtion
class CreateSingletonRecords < ActiveRecord::Migration[5.2]
  create_table :balance_holders do |t|
    t.string :type
    t.index :type, unique: true

    t.timestamps
  end
end

several methods in you parent class:

class SingletonRecord < ApplicationRecord
  class << self
    def instance
      @singleton__instance__
    end

    def load_record(params = {})
      @singleton__instance__ = find_or_create_by!(params)
    end
  end

  load_record

  validates :type, uniqueness: true
end

After it you can forever use single record for singleton model class. Your instance will be loaded or created one time durring model class loaded.

Answer 3

I am not sure I'd waste the database/ActiveRecord/Model overhead for such a basic need. This data is relatively static (I am assuming) and on the fly calculations aren't necessary (including database lookups).

Having said that, I'd recommend you define a YAML file with your site-wide settings and define an initializer file that loads the settings into a constant. You won't have nearly as many of the unnecessary moving parts.

There is no reason that data couldn't just sit in memory and save you a ton of complexity. Constants are available everywhere, and they don't need to be initialized or instantiated. If its absolutely critical that you utilize a class as a singleton, I'd recommend doing these two things:

1. undef the initialize/new method
2. define only self.* methods that way it is not possible for you to maintain a state

Answer 4

I know this is an old thread, but I just needed the same thing and found out that there's a gem for this: acts_as_singleton.

Installation instructions are for Rails 2, but it works great with Rails 3 too.

Answer 5

Using has_many :contacts doesn't mean you need a model. has_many does some magic, but in the end it's just adds some method with a specified contract. There's no reason why you can't implement those methods (or some subset that you need) to make your model behave like it has_many :contacts yet not actually use an ActiveRecord model (or model at all) for Contact.

Answer 6

You can do it like this:

 class Config < ApplicationRecord
  def self.instance
    Config.first || Config.create!
  end
 end