Riffing on Carlos's video https://www.youtube.com/watch?v=PjKtaQC4GLE

my_orm.rb

require 'yaml'

class MyOrm
  attr_accessor :id, :destroyed

  def save
    !!self.class.write_new(self)
  end

  def destroy
    !!self.class.remove(id)
  end

  def self.create(attributes = {})
    record = self.new(attributes)
    record.destroyed = false
    record.id = next_id
    record.save
    record
  end

  def self.first
    all.first
  end

  def self.last
    all.last
  end

  def self.count
    all.count
  end

  def self.remove(id)
    if (object = find_by(id: id))
      object.destroyed = true
      objects = all
      index = index_of(id)
      objects[index] = object
      write(objects)
    else
      false
    end
  end

  def self.destroy_all
    objects = all.map do |object|
      object.destroyed = true
      object
    end
    write(objects)
    objects.count
  end

  def self.index_of(id)
    all.index do |object|
      object.id == id
    end
  end

  def self.next_id
    if (last_record = self.all_including_destroyed.last)
      last_record.id + 1
    else
      1
    end
  end

  def self.all
    all_including_destroyed.reject(&:destroyed?)
  end

  def self.all_including_destroyed
    YAML.load(raw_data) || []
  end

  def destroyed?
    destroyed
  end

  def self.find_by(attributes)
    all.detect do |object|
      attributes.all? do |key, value|
        object.send(key) == value
      end
    end
  end

  def self.write_new(object)
    write(all_including_destroyed.push(object))
  end

  def self.write(objects)
    File.open(filename, "w") do |f|
      f.write YAML.dump(objects)
    end
  end

  def self.raw_data
    File.read(filename)
  rescue Errno::ENOENT
    ""
  end

  def self.filename
    "#{self.name.to_s.downcase}.yml"
  end
  def inspect
    "#<#{self.class.name} " + instance_variables.map do |ivar|
      "#{ivar}=#{instance_variable_get(ivar)}" unless [:@destroyed].include?(ivar)
    end.compact.join(' ') + ">"
  end
end

class Thundercat < MyOrm
  attr_reader :name

  def initialize(name: 'Snarf')
    @name = name
  end
end