base_repo.rb

class BaseRepo
  def initialize(csv_file)
    @csv_file = csv_file
    @elements = []
    @next_id = 1
    load_csv if File.exist?(@csv_file)
  end

  def create(element)
    element.id = @next_id
    @elements << element
    @next_id += 1
    save_csv
  end

  def all
    @elements
  end

  def find(id)
    @elements.find { |element| element.id == id }
  end

  def save_csv
    CSV.open(@csv_file, "wb") do |csv|
      csv << @elements.first.class.headers
      @elements.each do |element|
        csv << write_to_csv_row(element)
      end
    end
  end

  def load_csv
    options = { headers: :first_row, header_converters: :symbol }

    CSV.foreach(@csv_file, options) do |row|
      build_csv_row(row)
      @next_id = row[:id]
    end
    @next_id = @elements.empty? ? 1 : @elements.last.id + 1
  end
end

customer_repository.rb

require_relative "base_repo"
require_relative "../models/customer"

class CustomerRepository < BaseRepo
  def write_to_csv_row(element)
    [element.id, element.name, element.address]
  end

  def build_csv_row(row)
    row[:id] = row[:id].to_i
    @elements << Customer.new(row)
  end
end

meal_repo.rb

require_relative "base_repo"
require_relative "../models/meal"

class MealRepo < BaseRepo
  def write_to_csv_row(element)
    [element.id, element.name, element.price]
  end

  def build_csv_row(row)
    row[:id] = row[:id].to_i
    row[:price] = row[:price].to_i
    @elements << Meal.new(row)
  end
end