RDF module can't handle compound fields
>>> [!note] Migrated issue
<!-- Drupal.org comment -->
<!-- Migrated from issue #1778194. -->
Reported by: [linclark](https://www.drupal.org/user/396253)
>>>
<h2 id="summary-problem-motivation">Problem/Motivation</h2>
<p>Generally, vocabularies like Schema.org break things down into atomic units of data. Each unit contains a value that has a primitive data type; e.g. integer, plain text string. For example, when you have a schema:address, it isn't just one big HTML blob. Instead, you point to a PostalAddress item which then has properties of it's own; e.g. schema:streetAddress, addressLocality, addressRegion.</p>
<p>While you can model your fields exactly this way in Drupal using Field Collection, most people use fields like AddressField instead. AddressField does not represent the address as an entity reference, but instead as a field with properties. These properties are then assembled in the field formatter to create an HTML blob that represents the address. I have termed these fields <a href="http://lin-clark.com/blog/microdata-drupal-challenges-field-formatters#-but-more-work-isnt-necessarily-bad">compound fields</a>.</p>
<p>We don't have a way to represent these compound fields in the RDFa.</p>
<h3>RDF mapping API</h3>
<p>The RDF mapping API only provides the ability to map fields to RDF terms... the properties of those fields cannot be mapped.</p>
<pre><span style="color: #000000"><span style="color: #0000BB"><?php<br></span><span style="color: #007700">array(<br> array(<br> </span><span style="color: #DD0000">'type' </span><span style="color: #007700">=> </span><span style="color: #DD0000">'node'</span><span style="color: #007700">,<br> </span><span style="color: #DD0000">'bundle' </span><span style="color: #007700">=> </span><span style="color: #DD0000">'event'</span><span style="color: #007700">,<br> </span><span style="color: #DD0000">'mapping' </span><span style="color: #007700">=> array(<br> </span><span style="color: #DD0000">'rdftype' </span><span style="color: #007700">=> array(</span><span style="color: #DD0000">'schema:Event'</span><span style="color: #007700">),<br> </span><span style="color: #DD0000">'field_address' </span><span style="color: #007700">=> array(<br> </span><span style="color: #DD0000">'predicates' </span><span style="color: #007700">=> array(</span><span style="color: #DD0000">'schema:location'</span><span style="color: #007700">),<br> </span><span style="color: #DD0000">'type' </span><span style="color: #007700">=> </span><span style="color: #DD0000">'rel'</span><span style="color: #007700">,<br> </span><span style="color: #FF8000">// No specified way to add mappings for field properties here.<br> </span><span style="color: #007700">),<br> ),<br> ),<br>);<br></span><span style="color: #0000BB">?></span></span></pre><h3>Placement of attributes</h3>
<p>In addition to having no way to map the terms, there isn't an easy way for field formatter developers to place the RDFa correctly. Currently, there is a theme function called rdf_template_variable_wrapper, but there aren't any modules in core and only one in contrib (as of April) that use it. In order to use this theme function, you have to have the array of attributes to pass to it, which means you have to run the mapping through a conversion before calling the theme function.</p>
<p>In addition to being a relatively confusing API pattern, passing each property through a theme function from within the formatter seems unnecessarily heavy.</p>
<h2 id="summary-proposed-resolution">Proposed resolution</h2>
<p>Microdata module solves these two problems by: </p>
<ol>
<li>allowing nesting within a mapping</li>
<li>processing the mapping to an array of attributes, which it passes to the formatter. This means the formatter can place the attributes directly.</li>
</ol>
<p><strong>Example mapping:</strong></p>
<pre><span style="color: #000000"><span style="color: #0000BB"><?php<br></span><span style="color: #007700">array(<br> </span><span style="color: #DD0000">'node' </span><span style="color: #007700">=> array(<br> </span><span style="color: #DD0000">'event' </span><span style="color: #007700">=> array(<br> </span><span style="color: #DD0000">'field_address' </span><span style="color: #007700">=> array(<br> </span><span style="color: #DD0000">'#itemtype' </span><span style="color: #007700">=> array(</span><span style="color: #DD0000">'http://schema.org/PostalAddress'</span><span style="color: #007700">),<br> </span><span style="color: #DD0000">'#itemprop' </span><span style="color: #007700">=> array(</span><span style="color: #DD0000">'address'</span><span style="color: #007700">),<br> </span><span style="color: #DD0000">'locality' </span><span style="color: #007700">=> array(<br> </span><span style="color: #DD0000">'#itemprop' </span><span style="color: #007700">=> array(</span><span style="color: #DD0000">'addressLocality'</span><span style="color: #007700">),<br> ),<br> ),<br> ),<br> ),<br>);<br></span><span style="color: #0000BB">?></span></span></pre><p><strong>Example formatter code:</strong></p>
<pre><span style="color: #000000"><span style="color: #0000BB"><?php<br>$variables</span><span style="color: #007700">[</span><span style="color: #DD0000">'microdata'</span><span style="color: #007700">] = </span><span style="color: #0000BB">$entity</span><span style="color: #007700">-></span><span style="color: #0000BB">microdata</span><span style="color: #007700">[</span><span style="color: #0000BB">$field</span><span style="color: #007700">[</span><span style="color: #DD0000">'field_name'</span><span style="color: #007700">]];<br></span><span style="color: #0000BB">?></span></span></pre><p>In this example, it gets passed into the formatter's theme function, which places it</p>
<pre><span style="color: #000000"><span style="color: #0000BB"><?php<br>$locality_attributes </span><span style="color: #007700">= </span><span style="color: #0000BB">drupal_attributes</span><span style="color: #007700">(</span><span style="color: #0000BB">$microdata</span><span style="color: #007700">[</span><span style="color: #DD0000">'locality'</span><span style="color: #007700">][</span><span style="color: #DD0000">'#attributes'</span><span style="color: #007700">]);<br></span><span style="color: #0000BB">$output </span><span style="color: #007700">.= </span><span style="color: #DD0000">"<span </span><span style="color: #0000BB">$locality_attributes</span><span style="color: #DD0000">>Locality text here</span>"</span><span style="color: #007700">;<br></span><span style="color: #0000BB">?></span></span></pre>
> Related issue: [Issue #1778226](https://www.drupal.org/node/1778226)
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