Allow for empty mappings (work around pending core fix)

I hope this to get fixed soon, if it's related to http://drupal.org/node/1271498 (I cannot delete mappings and vocabularies)

Please give a hint how to empty/reset the rdf_mapping table. thanks
just delete the records, or the site will break?

+1

Posted a patch to fix core, works like this:

  $default_mapping = _rdf_get_default_mapping($mapping['type']);
  $only_in_default = array_diff($default_mapping, $mapping['mapping']);
  $mapping['mapping'] += $only_in_default;

This is what I do in rdf_mapping_save():

  $mapping['mapping'] += _rdf_get_default_mapping($mapping['type']);
  $mapping['mapping'] = array_filter($mapping['mapping']);

So any value in $mapping['mapping'] that evaluates to FALSE now means that no mapping is desired for the corresponding key. Use of operator += prevents those from being overridden by the default mappings, and in the end the FALSE values get filtered out.

Actually, my "fix" only works on the first save. The second time the mappings happen to get loaded and saved, the defaults find their way back into the mappings. Perhaps we could have a new special attribute type 'none' to explicitly mean "no mapping". I have not tested this idea, but it looks like the core RDF module will simply reject mappings whose attribute type is neither 'property', 'rel', or 'rev'.

Okay, so to get rid of default mappings, I'm now defining overrides like this:

<?php
  $rdf_mappings['node']['my_node_type'] = array(
    'name' => array(
      'predicates' => array(),
      'type' => '',
    ),
  );
?>

Except for a PHP notice in rdf.module's function rdf_rdfa_attributes() (which can be fixed with the attached patch), things seem to work smoothly.
A fork of rdf_mapping_save() is avoided with this approach.