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Querying OM Documents

This document will help you understand how to access the information associated with an OM::XML::Document object. We will explain some of the methods provided by the OM::XML::Document module and its related modules OM::XML::TermXPathGenerator & OM::XML::TermValueOperators

Note: In your code, don’t worry about including OM::XML::TermXPathGenerator and OM::XML::TermValueOperators into your classes. OM::XML::Document handles that for you.

Load the Sample XML and Sample Terminology

These examples use the Document class defined in OM::Samples::ModsArticle

Download hydrangea_article1.xml (sample xml) into your working directory, then run this in irb:

  require "om/samples"
  sample_xml = File.new("hydrangea_article1.xml")
  doc = OM::Samples::ModsArticle.from_xml(sample_xml)

Querying the OM::XML::Document

The OM::XML::Terminology" declared by OM::Samples::ModsArticle maps the defined Terminology structure to xpath queries. It will also run the queries for you in most cases.

xpath_for method of OM::XML::Terminology" retrieves xpath expressions for OM terms

The xpath_for method retrieves the xpath used by the OM::XML::Terminology"

Examples of xpaths for :name and two variants of :name that were created using the :ref argument in the Terminology builder:

OM::Samples::ModsArticle.terminology.xpath_for(:name)
=> "//oxns:name"
OM::Samples::ModsArticle.terminology.xpath_for(:person)
=> "//oxns:name[@type=\"personal\"]" 
OM::Samples::ModsArticle.terminology.xpath_for(:organization)
=> "//oxns:name[@type=\"corporate\"]"

Working with Terms

To retrieve the values of xml nodes, use the term_values method:

doc.term_values(:person, :first_name) 
doc.term_values(:person, :last_name)

The term_values method is defined in the OM::XML::TermValueOperators module, which is included in OM::XML::Document

Not that if a term’s xpath mapping points to XML nodes that contain other nodes, the response to term_values will be Nokogiri::XML::Node objects instead of text values:

  doc.term_values(:name)

More examples of using term_values and find_by_terms (defined in OM::XML::Document):

doc.find_by_terms(:organization).to_xml
doc.term_values(:organization, :role)
=> ["\n      Funder\n    "] 
doc.term_values(:organization, :namePart)
=> ["NSF"]

To retrieve the values of nested terms, create a sequence of terms, from outermost to innermost:

OM::Samples::ModsArticle.terminology.xpath_for(:journal, :issue, :pages, :start)
=> "//oxns:relatedItem[@type=\"host\"]/oxns:part/oxns:extent[@unit=\"pages\"]/oxns:start" 
doc.term_values(:journal, :issue, :pages, :start)
=> ["195"]

If you get one of the term names wrong in the sequence, OM will tell you which one is causing problems. See what happens when you put :page instead of :pages in your argument to term_values.

doc.term_values(:journal, :issue, :page, :start)
OM::XML::Terminology::BadPointerError: You attempted to retrieve a Term using this pointer: [:journal, :issue, :page] but no Term exists at that location. Everything is fine until ":page", which doesn't exist.

When XML Elements are Reused in a Document

(Another way to put this: the xpath statement for a term can be ambiguous.)

In our MODS document, we have two distinct uses of the title XML element:

  1. the title of the published article
  2. the title of the journal it was published in.

How can we distinguish between these two uses?

doc.term_values(:title_info, :main_title)
=> ["ARTICLE TITLE", "VARYING FORM OF TITLE", "TITLE OF HOST JOURNAL"] 
doc.term_values(:mods, :title_info, :main_title)
=> ["ARTICLE TITLE", "VARYING FORM OF TITLE"]
OM::Samples::ModsArticle.terminology.xpath_for(:title_info, :main_title)
=> "//oxns:titleInfo/oxns:title"

The solution: include the root node in your term pointer.

OM::Samples::ModsArticle.terminology.xpath_for(:mods, :title_info, :main_title)
=> "//oxns:mods/oxns:titleInfo/oxns:title"
doc.term_values(:mods, :title_info, :main_title)
=> ["ARTICLE TITLE", "VARYING FORM OF TITLE"]

We can still access the Journal title by its own pointers:

doc.term_values(:journal, :title_info, :main_title)
 => ["TITLE OF HOST JOURNAL"]

Making life easier with Proxy Terms

If you use a nested term often, you may want to avoid typing the whole sequence of term names by defining a proxy term.

As you can see in OM::Samples::ModsArticle, we have defined a few proxy terms for convenience.

t.publication_url(:proxy=>[:location,:url])
t.peer_reviewed(:proxy=>[:journal,:origin_info,:issuance], :index_as=>[:facetable])
t.title(:proxy=>[:mods,:title_info, :main_title])
t.journal_title(:proxy=>[:journal, :title_info, :main_title])

You can use proxy terms just like any other term when querying the document.

OM::Samples::ModsArticle.terminology.xpath_for(:peer_reviewed)
=> "//oxns:relatedItem[@type=\"host\"]/oxns:originInfo/oxns:issuance"
OM::Samples::ModsArticle.terminology.xpath_for(:title)
=> "//oxns:mods/oxns:titleInfo/oxns:title" 
OM::Samples::ModsArticle.terminology.xpath_for(:journal_title)
=> "//oxns:relatedItem[@type=\"host\"]/oxns:titleInfo/oxns:title"