A singleton module that tries to enforce that the first object being assigned as the singleton

singleton.cr

class Ytdb::Database
  SCHEMA_VERSION = 1_i64
  @@singleton : self? = nil
  property db : DB::Database
  property dbfile : String

  def initialize(@dbfile)
    @db = DB.open "sqlite3:#{@dbfile}?max_idle_pool_size=3&initial_pool_size=3"
    @@singleton ||= self
  end

  def self.singleton
    @@singleton || raise "#{self}: no singleton defined yet"
  end

  def self.singleton=(db : self)
    @@singleton = db
  end