Workaround for embedded documents in shrine-mongoid

script.rb

require "mongoid"
require "shrine"
require "shrine/storage/memory"
require "stringio"

Mongoid.configure do |config|
  config.clients.default = { hosts: ['localhost:27017'], database: 'my_db' }
  config.log_level = :debug
end

Shrine.storages = {
  cache: Shrine::Storage::Memory.new,
  store: Shrine::Storage::Memory.new,
}

Shrine.plugin :mongoid

module EmbeddedMongoidSupport
  module AttacherClassMethods
    def load_record(data)
      if data["parent"]
        parent_class, parent_id, association_name = data["parent"]
        child_class, child_id = data["record"]

        parent_class = Object.const_get(parent_class)
        parent       = find_record(parent_class, parent_id)
        association  = parent.send(association_name)

        association.where(id: child_id).first
      else
        super
      end
    end
  end

  module AttacherMethods
    def dump
      hash = super
      if record.embedded?
        hash["parent"] = [
          record._parent.class.to_s,
          record._parent.id.to_s,
          record.association_name.to_s
        ]
      end
      hash
    end

    def swap(new_file)
      if record.embedded?
        association = record._parent.send(record.association_name)
        reloaded = association.where(id: record.id).first
        return if reloaded.nil? || self.class.new(reloaded, name).read != read
        update(new_file)
      else
        super
      end
    end
  end
end

class ImageUploader < Shrine
  plugin :backgrounding
  plugin EmbeddedMongoidSupport

  Attacher.promote { |data| self.class.promote(data) } # mimic background job
end

class Album
  include Mongoid::Document
  embeds_many :photos
end

class Photo
  include Mongoid::Document
  include ImageUploader::Attachment.new(:image)

  embedded_in :album

  field :image_data, type: String
  field :caption,    type: String
end

album = Album.create
photo = album.photos.create(image: StringIO.new)
photo = album.reload.photos.first
photo.image # => #<ImageUploader::UploadedFile:0x00007f894e14fbc0 @data={"id"=>"c43279e81ccc51d6743a884ea9bd34b1", "storage"=>"store", "metadata"=>{"filename"=>nil, "size"=>0, "mime_type"=>nil}}>

photo.caption = "original caption"
photo.save

same_album = Album.last
same_photo = same_album.photos.last
same_photo.caption = "edited caption"
same_photo.save

this_photo_again = album.photos.where(id: same_photo.id).first
this_photo_again.caption # => "original caption"
this_photo_again.reload
this_photo_again.caption # => "edited caption"