Example read and append CSV ActiveModel implementation for Rails

csv_file_model.rb

# frozen_string_literal: true

require 'active_model'
require 'active_support/inflector'
require 'csv'

class CSVFileModel
  class RecordNotFound < StandardError
    def message = 'Record not found.'
  end

  include ActiveModel::API
  include ActiveModel::Attributes
  include ActiveModel::Dirty

  class << self
    def inherited(subclass)
      headers(subclass).each do |header|
        subclass.attribute(header, :string)
        validates header, presence: true
      end

      super
    end

    def headers(subclass = self) = CSV.foreach(filename(subclass)).first

    def filename(klass = self)
      klass
        .name
        .underscore
        .pluralize
        .concat('.csv')
    end

    def find_by(**attributes)
      found = CSV.foreach(filename, headers: true).find do |row|
        attributes.all? do |attribute, value|
          row[attribute.to_s] == value.to_s
        end
      end
      return unless found

      new(**found, &:persist!)
    end

   def find_by!(...)
      record = find_by(...)
      raise RecordNotFound unless record
      
      record
    end
  end

  attribute_method_suffix '?'

  def attribute?(field) = public_send(field).present?

  def save
    return true if persisted?

    self.id ||= next_id
    return false unless valid?

    append_csv

    self
  end

  def save!
    return true if persisted?

    self.id ||= next_id
    validate!

    append_csv

    self
  end

  def pretty_print(pp)
    pp.object_address_group self do
      pp.breakable
      pp.seplist(attributes) do |field, value|
        pp.text "#{field}=#{value.inspect}"
      end
    end
  end

  def persist!
    changes_applied

    @persisted = true
  end

  def persisted? = @persisted
  
  private

  attr_accessor :persisted

  def next_id
    CSV.foreach(self.class.filename, headers: true)
       .reverse_each
       .first
       .fetch('id', -1)
       .to_i
       .succ
  end  
  
  def append_csv
    CSV.open(self.class.filename, 'a+', headers: true) do |csv|
      csv << attributes.values
    end

    persist!
  end
end

class Student < CSVFileModel
end

student = Student.new
student.name = 'shan'
student.teacher_name = 'romy'
student.subject = 'typing'
pp student.changes

student.save!
pp student
pp student.changes

pp Student.find_by(id: 4)

students.csv