class Picture < ActiveRecord::Base

  ...

  has_attached_file :image, styles: { medium: "400x400>", thumb: "100x100>" }

  def diff(picture)
    myself = Magick::Image.read(Paperclip.io_adapters.for(self.image.styles[:medium]).path).first
    target = Magick::Image.read(Paperclip.io_adapters.for(rank.image.styles[:medium]).path).first
    myself = myself.resize_to_fit(200)
    target = target.resize_to_fit(200)
    diff = myself.composite(target, 0, 0, Magick::DifferenceCompositeOp)
  end

  def same?(picture)
    diff = self.diff(picture)
    diff.channel_mean.first.to_i <= 5000
  end

  ...

  end