Question

I have a need to compress large chunk of text before saving it to the database and decompress it back once client requests it.

The method I am using right now seems to work fine when I insert new records using the Rails console and query for the newly inserted record right away. i.e., I can decompress the compressed description successfully.

But I am not able to decompress the compressed description for any of my other records added prior to this date. It is really confusing for me especially being a beginnner to the ROR world.

I am using MySQL as a database.

See my Model below to better understand it.

require "base64"

class Video < ActiveRecord::Base
  before_save :compress_description

  def desc
    unless description.blank?
      return decompress(description)
    end
  end

  private

  def compress_description
    unless description.blank?
      self.description = compress(description)
    end
  end

  def compress(text)
    Base64.encode64(Zlib::Deflate.new(nil, -Zlib::MAX_WBITS).deflate(text, Zlib::FINISH))
  end

  def decompress(text)
    Zlib::Inflate.new(-Zlib::MAX_WBITS).inflate(Base64.decode64(text))
  end
end

Answer 1

Ok it’s actually very easy to reproduce your problem. In rails console do the following

Video.create(:description => "This is a test")
Video.last.description
=> "C8nILFYAokSFktTiEgA=\n" 
Video.last.desc
=> "This is a test" 
Video.last.save #This update corrupts the description
Video.last.desc
=> "C8nILFYAokSFktTiEgA=\n"

The reason the corruption happens is because you are compressing an already compressed string

You should probably modify your class as follows and you should be fine

require 'base64'
class Video < ActiveRecord::Base
  before_save :compress_description
  after_find :decompress_description
  attr_accessor :uncompressed_description

  private

  def compress_description
    unless @uncompressed_description.blank?
    self.description = compress(@uncompressed_description)
    end
  end

  def decompress_description
    unless description.blank?
      @uncompressed_description = decompress(description)
    end
  end

  def compress(text)
    Base64.encode64(Zlib::Deflate.new(nil, -Zlib::MAX_WBITS).deflate(text, Zlib::FINISH))
  end

  def decompress(text)
    Zlib::Inflate.new(-Zlib::MAX_WBITS).inflate(Base64.decode64(text))
  end
end

Now use your class as follows

Video.create(:uncompressed_description => "This is a test")
Video.last.description
=> "C8nILFYAokSFktTiEgA=\n" 
Video.last.uncompressed_description
=> "This is a test" 
Video.last.save
Video.last.uncompressed_description
=> "This is a test"