NullObject

comment.rb

class Comment < ActiveRecord::Base
  delegate :email, to: author, prefix: true, allow_nil: true
end

null_object.rb

class NullObject
  def present?; false; end
  def empty?; true; end
  def nil?; true; end
  def !; true; end
  def tap; self; end
  def to_a; []; end
  def to_s; ""; end
  def to_f; 0.0; end
  def to_i; 0; end
  def method_missing(*args, &block)
    if args.first.to_s[/\?/]
      raise NoMethodError, <<-HEREDOC.squish
        #{args.inspect}
        on: #{to_yaml}
      HEREDOC
    end
    nil
  end
end

null_object_persistable.rb

module NullObjectPersistable
  extend ActiveSupport::Concern

  included do
    def self.mimics_persistence_from(real_model_class)
      @real_model_class = real_model_class
    end

    def self.real_model_class
      @real_model_class
    end

    def self.table_name
      @real_model_class.to_s.tableize
    end

    def self.primary_key
      'id'
    end
  end

  def real_model_class
    self.class.real_model_class
  end

  def id
  end

  def [](*)
  end

  def is_a?(klass)
    if klass == real_model_class
      true
    else
      super
    end
  end

  def destroyed?
    false
  end

  def new_record?
    false
  end

  def persisted?
    false
  end
end

null_user.rb

class NullUser
  include NullObjectPersistable

  mimics_persistence_from User
end