Cannot sort by Taxonomy fields with Solr 3.5

For fields, we index every first field as a single field also, so we can easily have sorts and multivalued fields.

The problem here is that a field that is assigned to only have 1 value, can easily be switched to a multivalued field in Drupal and solr would have to discard all his data. We already do this for default fields, so I'm ok with doing this for terms also.

Any other comments?

Sorting by taxonomy terms does seems to be a bit of an edge case. When would this be useful?

Seems more people has hit this bug. Marking it as RC blocker + bug

I still don't see the use case. If we want a single value for sorting only, maybe it should just be copied to ss_sort_* in Drupal?

copying it to ss_sort_* would make sense to me, just hoping that it doesn't confuse users too much.
I'm also ok with just educating site builders that they cannot sort on multivalued fields and that they should use a little custom coding to make this work. We could even move this to the apachesolr_sort module where you could select which fields you need to have sortable and the module would do the rest magically

   <!-- Sortable version of the dynamic string field -->
   <dynamicField name="sort_*" type="sortString" indexed="true" stored="false"/>
   <copyField source="ss_*" dest="sort_*"/>

So every field in ss_* is copied to the exact same field in sort_*
ss_mytaxo = sort_mytaxo

This way you can enable sorts for the sort_mytaxo field and you should be good to go. Be aware that this conversion does not happen for the sm_* field since you cannot sort on multiple! You could, using some mapping alter convert your specific taxo field to a single field

/**
 *
 * @param type $mappings
 * @param type $entity_type
 */
function HOOK_apachesolr_field_mappings_alter(&$mappings, $entity_type) {
  // Add our per field mapping here so we can sort on the
  // taxonomy field by making it single. Solr cannot sort on multivalued fields
  // taxo_ref is our identifier of a custom field. Do check that this field is the correct name. 
  // We use the same properties as the list_text
  $mappings['per-field']['taxo_ref'] = $mappings['list_text'];
  // We set multiple explicitly as FALSE!
  $mappings['per-field']['taxo_ref']['multiple'] = FALSE;
}

Marking this as fixed until someone will reopen with a more ui friendly way of fixing this. But I hope some custom coding would suffice.

If you don't need the single value field e.g. for faceting, you might want to just copy on the Drupal side to a sort_ field.