YAML and ruby classes

Ruby
1

I need to create some objects of different (custom) classes, in
different pages of my site, so I made some code that takes an array of
hashes and create the objects. the hashes are like this one:

h = { :class => Class1, :param1 => ‘foo’, :param2 => ‘bar’, etc }

and basically I do

object = h[:class].new(h)

everything works fine as long as the hash is defined inside the source
code. however in some case I need to create the same object in two
different file. to avoid writing the same hashes twice I though about
putting them in a YAML file but this method doesn’t work anymore because
the class name is converted to a string instead of a reference to the
class, using !ruby/object creates the object but all the code inside the
initialize method seems to be never executed (or the instance variables
ovverrided after the inzialize method) so my objects don’t work,
ClassName.to_yaml returns an error, etc…

is there a way to do what I want to do? putting the ashes in a file and
loading them as a source AFAIK creates other problem due to the
sandboxing made by mod_ruby, that’s why I tried with yaml.

2

Tirsdag den 13. Marts 2007 skrev Matteo C.:

everything works fine as long as the hash is defined inside the source
loading them as a source AFAIK creates other problem due to the
sandboxing made by mod_ruby, that’s why I tried with yaml.

Object.const_get can convert a String to a constant, so if you do:

  Object.const_get(h[:class].to_s).new(h)

it should work, no matter if the hash comes from source or yaml

br. Chr.

3

On Mar 13, 2007, at 9:50 AM, Matteo C. wrote:

everything works fine as long as the hash is defined inside the source
ClassName.to_yaml returns an error, etc…

is there a way to do what I want to do? putting the ashes in a file
and
loading them as a source AFAIK creates other problem due to the
sandboxing made by mod_ruby, that’s why I tried with yaml.

It seems to me this is a fundamental problem with YAML. Even if you
were trying to put all the data in one file, I don’t believe the YAML
spec. addresses writing object references instead of objects. This is
especially an issue when your objects have circular references.

This caused me to use XML instead of YAML for a recent Java project
because the Java XStream library handles serializing and
deserializing Java objects that have circular references.

If there is a YAML solution to this, I’d love to hear about it!

4

On Wed, 14 Mar 2007, Mark V. wrote:

It seems to me this is a fundamental problem with YAML. Even if you were
trying to put all the data in one file, I don’t believe the YAML spec.
addresses writing object references instead of objects. This is especially an
issue when your objects have circular references.

This caused me to use XML instead of YAML for a recent Java project because
the Java XStream library handles serializing and deserializing Java objects
that have circular references.

If there is a YAML solution to this, I’d love to hear about it!

harp:~ > ruby -r yaml -e’ h = {}; h[:h] = h; y h ’
&id001
:h: *id001

regards.

-a

5

On Mar 13, 2007, at 10:28 AM, [email protected] wrote:

This caused me to use XML instead of YAML for a recent Java
project because the Java XStream library handles serializing and
deserializing Java objects that have circular references.

If there is a YAML solution to this, I’d love to hear about it!

harp:~ > ruby -r yaml -e’ h = {}; h[:h] = h; y h ’
&id001
:h: *id001

My apologies! Apparently the problem is with the Java implementation
of YAML that I was using and not with YAML itself. Here’s a more full
example that demonstrates YAML doing the right thing with multiple
references to the same object and with circular references.

require ‘yaml’

class Person
attr_accessor :name, :spouse, :address

def to_s
“\n#{name} is married to #{spouse.name} and lives at\n#{address}”
end
end

class Address
attr_accessor :street, :city, :state, :zip

def to_s
“#{street}\n#{city}, #{state} #{zip}”
end
end

a = Address.new
a.street = “644 Glen Summit”
a.city = “St. Charles”
a.state = “MO”
a.zip = 63304

p1 = Person.new
p1.name = “Mark V.”
p1.address = a

p2 = Person.new
p2.name = “Tami Volkmann”
p2.address = a

p1.spouse = p2
p2.spouse = p1

people = [p1, p2]
yaml_string = YAML::dump(people)
puts yaml_string

new_people = YAML::load(yaml_string)
puts new_people

The output is

  • &id002 !ruby/object:Person
    address: &id001 !ruby/object:Address
    city: St. Charles
    state: MO
    street: 644 Glen Summit
    zip: 63304
    name: Mark V.
    spouse: &id003 !ruby/object:Person
    address: *id001
    name: Tami Volkmann
    spouse: *id002
  • *id003

Mark V. is married to Tami Volkmann and lives at
644 Glen Summit
St. Charles, MO 63304

Tami Volkmann is married to Mark V. and lives at
644 Glen Summit
St. Charles, MO 63304

6

Christian S. [email protected] wrote:

Object.const_get can convert a String to a constant, so if you do:

  Object.const_get(h[:class].to_s).new(h)

it should work, no matter if the hash comes from source or yaml

the idea was excellent but it seems it doens’t work if the class I need
is defined in another class namespace, e.g.

irb(main):117:0> Object.const_get(‘CGI::Cookie’)
NameError: wrong constant name CGI::Cookie

anyway thanks for the help

7

Mark V. [email protected] wrote:

This caused me to use XML instead of YAML for a recent Java project

mh… if I don’t find a solution with yaml I’ll try xml with ruby.
thanks.

8

On Tue, 13 Mar 2007 15:47:13 +0100, Matteo C. wrote:

everything works fine as long as the hash is defined inside the source
loading them as a source AFAIK creates other problem due to the
sandboxing made by mod_ruby, that’s why I tried with yaml.

If you reference the class object directly, you can’t dump it to yaml,
so
try using a string instead, and running eval on the string to get the
class object.

h = { :class => ‘Class1’, :param1 => ‘foo’, :param2 => ‘bar’, etc }
object = eval(h[:class]).new(h)

This gives you the following YAML:

:class: Class1
:param1: foo
:param2: bar

9

Ken B. [email protected] wrote:

h = { :class => ‘Class1’, :param1 => ‘foo’, :param2 => ‘bar’, etc }
object = eval(h[:class]).new(h)

it works! thanks :slight_smile:

10

Stefano C. [email protected] wrote:

name.split(‘::’).inject(Object){|res, c| c.const_get(c)}.new h

this is pretty complicated for my current knowledge of ruby :smiley: the
method suggested by ken bloom works and is simpler for me, but thanks
anyway for you help! :slight_smile:

11

Alle martedì 13 marzo 2007, Matteo C. ha scritto:

irb(main):117:0> Object.const_get(‘CGI::Cookie’)
NameError: wrong constant name CGI::Cookie

anyway thanks for the help

Can’t you store the class name (using Class#name) instead of the class
itself
in the yaml file? This way, since Class#name returns the full ‘path’,
including modules, you can then do something like:

name=h[:class]
name.split(‘::’).inject(Object){|res, c| c.const_get(c)}.new h

(taken from the solutions of the ruby quiz 133
(Ruby Quiz - One-Liners (#113)))

I hope this helps

Stefano

12

Matteo C. wrote:

everything works fine as long as the hash is defined inside the source
loading them as a source AFAIK creates other problem due to the
sandboxing made by mod_ruby, that’s why I tried with yaml.

It is possible to extend YAML to serialize classes:

http://blade.nagaokaut.ac.jp/cgi-bin/scat.rb/ruby/ruby-talk/177604

It seems to still work with ruby-1.8.6.

It lets you do this:

yy = [Enumerable, Comparable, String, File].to_yaml
puts yy
p YAML.load(yy)

with output:

  • !ruby/module Enumerable
  • !ruby/module Comparable
  • !ruby/class String
  • !ruby/class File
    [Enumerable, Comparable, String, File]

However, if you are hand-editing the YAML file, you may find that a
class name string is more convenient than this notation.

YYAMLMMV

13

On Wed, Mar 14, 2007 at 01:25:06AM +0900, Matteo C. wrote:

Stefano C. [email protected] wrote:

name.split(‘::’).inject(Object){|res, c| c.const_get(c)}.new h

this is pretty complicated for my current knowledge of ruby :smiley: the
method suggested by ken bloom works and is simpler for me, but thanks
anyway for you help! :slight_smile:

To get at Foo::Bar::Baz you need to do

klass = Object.const_get("Foo").const_get("Bar").const_get("Baz")

So that one-liner above is really another way of writing:

name = “Foo::Bar::Baz”
names = name.split(“::”)
klass = Object
while n = names.shift
klass = klass.const_get(n)
end

14

Joel VanderWerf [email protected] wrote:

However, if you are hand-editing the YAML file, you may find that a
class name string is more convenient than this notation.

I think i’ll go for the class name string :slight_smile: but thanks for the link to
the extension. as I’m still learning ruby it was very interesting.