evil_vfs.rb

###
# This is a demonstration of using SQLite3's Virtual File System API in Ruby.
#
# == Synopsis
#
# This program will store its SQLite database after the __END__ line.
#
# === In Detail
#
# SQLite3 uses the DATABase class as a proxy for our IO object.  Upon
# instantiation, the DATABase class opens __FILE__, seeks to the point right
# after __END__ by using DATA.pos.  It caches the value of DATA.pos for later
# calculations when the database engine needs to seek for reading and writing.
#
# == How can I run this code?
#
# This code requires my fork of the sqlite3-ruby bindings.  The normal sqlite
# bindings do not expose the Virtual File System api.  You can get my fork
# with the VFS extensions here:
#
#   http://github.com/tenderlove/sqlite3-ruby/tree/vfs
#

require 'sqlite3'

class EvilVFS < SQLite3::VFS
  def open name, flags
    DATABase.new name, flags
  end
end

class DATABase < SQLite3::VFS::File
  def initialize name, flags
    super
    @store  = File.open(name, File::RDWR | File::CREAT)
    @offset = 0

    if File.expand_path(__FILE__) == name
      @store.seek DATA.pos
      @offset = DATA.pos
    end
  end

  def close
    @store.close
  end

  def read amt, offset
    @store.seek(offset + @offset)
    @store.read amt
  end

  def write data, offset
    @store.seek(offset + @offset)
    @store.write data
  end

  def sync opts
    @store.fsync
  end

  def file_size
    File.size(@store.path) - @offset
  end
end

SQLite3.vfs_register(EvilVFS.new)
db = SQLite3::Database.new(__FILE__, nil, 'EvilVFS')

db.execute('create table if not exists users(id integer primary key, name string)')
100.times { db.execute('insert into users(name) values (?)', 'tenderlove') }
p db.execute('select count(*) from users')

__END__