Subclasses with no additional nodes can't be loaded from XML

[dmolesUC]

I have a case where I want to create objects of different types from the same XML, depending on the value of certain attributes. I want to declare all the attributes in a superclass and then extend that for each subclass. Unfortunately, something in the mapping initialization seems to be dependent on declaring nodes in the leaf class. If I try to load_from_xml() on a leaf class with no nodes of its own, I get

/Users/dmoles/.rvm/gems/ruby-2.2.1/gems/xml-mapping-0.10.0/lib/xml/mapping/base.rb:343:in
    `load_from_xml': undefined mapping: :_default (XML::MappingError)

Clearly there's something going on by side effect when you declare a node, but I haven't figured out what it is.

Example:

#!/usr/bin/env ruby

require 'xml/mapping'

xml_str = '<foo bar="baz"/>'
xml = REXML::Document.new(xml_str).root

# ------------------------------
# No inheritance -- works

class Foo
  include XML::Mapping

  text_node :bar, '@bar', default_value: nil
end

foo = Foo.load_from_xml(xml)
puts "foo.bar: #{foo.bar}"

# ------------------------------
# Inheritance + defines an extra node -- works

class Bar < Foo
  include XML::Mapping

  text_node :baz, '@baz', default_value: nil
end

bar = Bar.load_from_xml(xml)
puts "bar.bar: #{bar.bar}"

# ------------------------------
# Inheritance + no nodes of its own -- fails 

class Baz < Foo
  include XML::Mapping
end

baz = Baz.load_from_xml(xml) # <- XML::MappingError

[dmolesUC]

Update: Found a simpler workaround -- see below.

---

As a workaround, I created a dummy node --

    class XmlPlaceholder < ::XML::Mapping::SingleAttributeNode
      def initialize(*args)
        path, *args = super(args[0], :placeholder, '@placeholder', { default_value: nil })
        @path = ::XML::XXPath.new(path)
        args
      end

      def extract_attr_value(_xml); end

      def set_attr_value(_xml, _value); end
    end

    ::XML::Mapping.add_node_class XmlPlaceholder

-- and included it in the subclasses:

class Baz < Foo
  include XML::Mapping
  xml_placeholder # Workaround for https://github.com/multi-io/xml-mapping/issues/4
end

[dmolesUC]

I found a simpler workaround: adding

use_mapping :_default

to the subclasses.

[dmolesUC]

Another workaround: use the inherited hook in the superclass to avoid having to add use_mapping to each subclass individually:

class Foo
  include XML::Mapping

  text_node :bar, '@bar', default_value: nil

  def self.inherited(base)
    base.use_mapping :_default
  end
end

Note that you'll need a use_mapping call for any alternate mappings in the superclass, as well.

[yevgenko]

@dmolesUC3, thank you for sharing workarounds, I was facing the same issue today

[dmolesUC]

@yevgenko No problem!