How can I create form object for the update action?

There is a form object which allows creating an object with associations. This main problem is associations update. I really don't know how to extend this without tons of each and conditions. What is the most convenient way to extend this class for updating object?

module Company
  class JobForm
    include ActiveModel::Model

    attr_accessor :title, :body, :city, :start_date, :attachments, 
                          :requirements, :company
    attr_reader :object

    validates :title, :body, :start_date, :city, presence: true

    def save
      persist! if valid?
    end

    private

    def persist!
      ActiveRecord::Base.transaction do
        save_job
        save_attachments
        save_requirements
      end
      object
    end

    def save_job
      @object = company.jobs.create( 
          # attributes.. ..)
    end

    def save_attachments
      return unless attachments&.present?
      attachments.each do |attacment_attributes|
        object.attachments.create(attacment_attributes)
      end
    end

    def save_requirements
      return unless requirements&.present?
      object.requirements.destroy_all
      requirements.each do |requirement_attributes|
        object.requirements.create(requirement_attributes)
      end
    end
  end
end