Adding helpful error messages to your Ruby code

jrobertson (James Robertson) — Wed, 30 Apr 2008 11:40:47 GMT

This Ruby code raises an error if the XPath query fails because the attribute being queried did not exist for the given element.



  def map_pattr(node, fieldx, valuex)

    begin

    parameter = node.root.elements["parameter[@field='#{fieldx}']"]

    parameter.add_attribute('value', valuex)

    parameter

    

    rescue

      puts 'feedpopulated.rb: map_attr() the field ' + fieldx + ' was not found in params.'

      raise

    end

  end

Notice that a raise statement is used to ensure that the system error message is raised and any further code execution is halted.

Without adding a customized helpful message I would be left scratching my head trying to work out what the following system error message meant.



./feedpopulated.rb:27:in `map_pattr': undefined method `add_attribute' for nil:NilClass (NoMethodError)

     from ./feedpopulated.rb:51:in `create_record'

     from ./feedpopulated.rb:49:in `each'

     from ./feedpopulated.rb:49:in `create_record'

     from ./recordx.rb:91:in `call_create'

     from ./s3fileuploader_handler.rb:14:in `call'

     from ./s3fileuploader_handler.rb:40:in `invoke'

     from ./uploadtwitteraudio.rb:22:in `initialize'

     from /usr/lib/ruby/1.8/rexml/element.rb:890:in `each'

     from /usr/lib/ruby/1.8/rexml/xpath.rb:53:in `each'

     from /usr/lib/ruby/1.8/rexml/element.rb:890:in `each'

     from ./uploadtwitteraudio.rb:18:in `initialize'

     from ./uploadtwitteraudio.rb:72:in `new'

     from ./uploadtwitteraudio.rb:72

Reference: Programming Ruby: The Pragmatic Programmer's Guide - Exceptions, Catch, and Throw [ruby-doc.org]

DZone Snippets: begin code

Adding helpful error messages to your Ruby code