A generic (flat) collection parser using Nokogiri's SAX capabilities

collection_parser.rb

require 'nokogiri'                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  
class CollectionParser < Nokogiri::XML::SAX::Document                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             
  attr_reader :results                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  
  # Initializes a generic SAX parser for extracting collections from XML                                                                                                                                                                                                                                                                                                                                                                                                                                                                          
  #                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               
  # @param item_tag [String]                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    
  # @param elems    [Hash<String=>[String]>]
  def initialize(item_tag, el_attrs)                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              
    @item_tag           = item_tag                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                
    @el_attrs           = el_attrs                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                
    @elems              = el_attrs.keys                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  
    @results            = []                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      
    @current_item       = nil                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
    @current_elem       = nil                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
    @current_elem_value = ""                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      
  end                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  
  def start_element(el, attr_pairs)                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               
    case el                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       
    when @item_tag                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                
      @current_item = {}                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          
    when *@elems                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  
      set_attributes(el, attr_pairs)                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              
      @current_elem = el                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          
    end                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           
  end                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  
  def end_element(el)                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             
    case el                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       
    when @item_tag                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                
      append_current_item                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         
    when *@elems                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  
      set_tag_value                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               
    end                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           
  end                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  
  def characters(cs)                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              
    append_text cs                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                
  end                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  
  def cdata_block(cs)                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             
    append_text cs                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                
  end                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  
  private                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  
  def append_current_item                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         
    @results << @current_item.dup                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 
    @current_elem = nil                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           
    @current_elem_value = ""                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      
  end                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  
  def set_tag_value                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               
    @current_item.merge!(@current_elem => @current_elem_value) if is_relevant_tag?                                                                                                                                                                                                                                                                                                                                                                                                                                                                
    @current_elem_value = ""                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      
  end                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  
  def set_attributes(el, attr_pairs)                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              
    return unless @current_item && !attr_pairs.empty?                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  
    attrs = attr_pairs.reduce({}) do |acc, (attr, v)|                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             
      if is_relevant_attribute?(el, attr)                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         
        acc.merge("#{el}_#{attr}" => v)·                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          
      else                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        
        acc                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       
      end                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         
    end                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           
    @current_item.merge!(attrs)                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   
  end                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  
  def is_relevant_attribute?(el, attr)                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            
    @el_attrs[el].include?(attr)                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  
  end                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  
  def is_relevant_tag?                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            
    @current_item && @current_elem                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                
  end                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  
  def append_text(cs)                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             
    @current_elem_value << cs if @current_elem                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    
  end                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             
end

example.rb

require_relative './collection_parser'                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            
require 'open-uri'
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  
el_attrs = {                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            
  'title'       => [],
  'link'        => [],                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                
  'guid'        => ['isPermaLink'],                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               
  'pubDate'     => [],                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            
  'description' => []                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             
}                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  
xml    = open("http://redit.com/r/bigdata.rss?limit=100").read                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              
feed   = CollectionParser.new('item', el_attrs)                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   
parser = Nokogiri::XML::SAX::Parser.new(feed)                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
parser.parse(xml)                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  
feed.results.last 
#=> {"title"=>"The Big Data Showstoppers: Frameworks", "link"=>"http://www.reddit.com/r/bigdata/comments/1x658q/the_big_data_showstoppers_frameworks/", "guid_isPermaLink"=>"true", "guid"=>"...", "pubDate"=>"Thu, 06 Feb 2014 02:31:07 -0800", "description"=>"..."}