Rails - Jul 2005 - Using paths to denote objects

In the past, in environments other than Rails, I''ve a couple of times 
gainfully used path expressions to refer to objects in the UI-layer. 
Now, in Rails, I''m already using them for creating lists where columns 
are described by meta data. I take it that there are even more an 
better uses. I haven''t thought this through carefully, so here are just
one example.

Currently, to get a text input field one writes something like this

<%= text_field ''address'', ''street'' %>

This requires the existence of a binding for ''address'' in the
template.
Now assume the address is reachable from another object,
''person'',
anyway. Then how about writing it like this instead:

<%= text_field ''person.address.street'' %>

which is meant to result in this HTML

<input type="text" id="person_address_street"
 name="person[address][street]" value="..." />

For iteration over multiple values a wildcard, ''*'', could be
used in the
path. For the moment I don''t quite see how to make this really useful, 
though.

I think a fairly minimal language for expressing paths is sufficient in 
Rails as there''s always Ruby to fall back on. Full-grown navigational 
language such as XPath-applied-to-objects or OGNL (Java) are more 
appropriate for environments where the host language is not accessible.

Below is my first stab at an extended Object#send method that handles 
paths. I''m interested in your thoughts.

Michael


class Object

  alias_method :send_without_path, :send
  
  def send(path, *args)
    if path.kind_of?(Symbol)
      return self.send_without_path(path, *args)
    elsif path.kind_of?(Array)
      path = path.dup
    elsif path.kind_of?(String)
      path = path.split(''.'')
    else
      raise TypeError, "Unrecognized path type for send:
<#{path}>"
    end
    
    method = path.shift
    
    case
    when method == ''*''
      raise TypeError, "Wildcard path item is only applicable to 
Enumerables; self: <#{self}>" unless self.kind_of?(Enumerable)
      if !path.empty?
        next_value = self.inject([]) { |result, elem|
          result << elem.send(path, *args)
        }
      else
        next_value = self
      end
      return next_value
    
    when self.respond_to?(method)
      next_value = self.send_without_path(method, *args)
    
    when self.respond_to?(''[]'')
      next_value = self.send_without_path(''[]'', method, *args)
      
    else
      raise TypeError, "Can''t advance path to
''#{method}''; no applicable
method for <#{self.inspect}>"
    end
    
    if not path.empty?
      next_value.send(path, *args)
    else
      next_value
    end
  end

end


-- 
Michael Schuerig                  Failures to use one''s frontal lobes
mailto:michael-q5aiKMLteq4b1SvskN2V4Q@public.gmane.org        can result in the
loss of them.
http://www.schuerig.de/michael/   --William H. Calvin