ActiveStorage NullAttachment

_active_storage_preview.html.slim

- if file.blob.image?
  = image_tag file.variant(resize: "512x512"), class: 'img-thumbnail w-25 float-left mr-3'
- elsif file.blob.content_type =~ /pdf/
  = image_tag file.preview(resize: "512x512"), class: 'img-thumbnail w-25 float-left mr-3'

active_storage_previewable.rb

module ActiveStoragePreviewable
  extend ActiveSupport::Concern

  included do
    def method_missing(method_name, *args, &block)
      if /(.*)_shim\Z/.match?(method_name)
        public_send(prefix(method_name)).attached? ? public_send(prefix(method_name)) : NullAttachment.new
      else
        super
      end
    end

    def respond_to_missing?(method_name, include_private = false)
      if /(.*)_shim\Z/.match?(method_name)
        attachments = reflect_on_all_associations.select do |assoc|
          assoc.klass == ActiveStorage::Attachment
        end

        attachments.map(&:name).map(&:to_s).include? "#{prefix(method_name)}_attachment"
      else
        super(method_name, include_private)
      end
    end

    private

    def prefix(method_name)
      (/(.*)_shim\Z/.match method_name.id2name)[1]
    end
  end
end

null_attachment.rb

class NullAttachment
  attr_reader :blob

  Blob = Struct.new(:content_type) do
    def image?
      content_type.start_with?("image")
    end
  end

  def initialize(content_type: 'image')
    @blob = Blob.new(content_type)
  end

  def variant(resize:)
    "http://placehold.it/#{resize}"
  end

  def preview(resize:)
    "http://placehold.it/#{resize}"
  end
end

show.html.slim

= render partial: 'common/active_storage_preview', locals: { file: @user.profile_picture_shim }

user.rb

class User < ApplicationRecord  
  include ActiveStoragePreviewable
  
  has_one_attached :profile_picture
end