rails g model AdminSettings --force-plural

class CreateAdminSettings < ActiveRecord::Migration[7.0]
  def change
    create_table :admin_settings do |t|
      t.boolean :singleton_guard, default: true, null: false

      t.timestamps
    end

    add_index :admin_settings, :singleton_guard, unique: true
  end
end

class AdminSettings < ApplicationRecord
  validates :singleton_guard, inclusion: { in: [true] }, uniqueness: true

  def self.instance
    first_or_create(singleton_guard: true)
  end
end

How to use

Now you can call AdminSettings.instance to fetch the object or create one.