So, following on from the original discussion-thread here: http://groups.google.com/group/rubyonrails-core/browse_thread/thread/8600da28a92d83ba
I've built some baseline functionality for defining a schema for an Active Resource.
Code available as a patch here: http://pastie.org/743842
and a more-readable combined diff here: http://pastie.org/743843
What you can now do is this:
class FlickrPhoto << ActiveResource::Base
define_schema do |s| s.string :photo, :title s.integer :width, :height s.float :some_other_attribute s.attribute 'and_another', 'string' end
validates_presence_of :photo validates_numericality_of :width, :height end
# this works my_photo = FlickrPhoto.new(:photo => 'photo_stream_here', :title => 'eben', :tags => ['cat']) my_photo.valid? # => true my_photo.save # => true my_photo.photo # 'photo_stream_here'
# and now, so does this: new_photo = FlickrPhoto.new() new_photo.respond_to? :photo # => true new_photo.photo # => nil new_photo.valid? # => false
new_photo.known_attributes # => ['photo', 'title', 'height', 'width', 'some_other_attribute', 'and_another']
# and if you fetch and existing one: a_photo = FlickrPhoto.find(1) # <FlickrPhoto><photo>abcdef</
<width>123</width><instance_attribute>456</instance_attribute></
a_photo.photo # => 'abcdef' a_photo.width # => '123' # note: still a string a_photo.instance_attribute # => '456' # note: also a string
a_photo.known_attributes # => ['photo', 'title', 'height', 'width', 'some_other_attribute', 'and_another', 'instance_attribute']
new_photo.respond_to? 'instance_attribute' # => false (because it's not a known_attribute)
More details on what/how in the rdoc for define_schema 
To summarise:
1) You can define a schema using 'define_schema' and passing a block,
or passing a hash to 'schema='
2) this populates a set of 'known_attributes' that are added to the
list of attributes currently on an instance (if there's no schema,
this means that the known_attributes equals the existing attributes on
the instance - as it always has done).
3) known attributes will return 'true' to a 'respond_to?' and will not
cause a 'NoMethodError' if invoked
4) it will also store the 'attribute type' against each attribute -
currently this does nothing... but my next step is to add a typecast
based on this, along with "attributes_before_typecast" ala Active
Record. This will be extremely useful for integer values as I've found
my own code peppered with "to_i" checks for attributes that I already
know are meant to be integers 
5) an extension will also allow us to pass in options. The only
sensible one that I can see so far is ":default => 'blah') which will,
of course, set up known attributes with their given defaults - unless
otherwise specified.
Cheers, Taryn
