Support multi-select facets with item counts independent of own facet

cosmetics

(From cpliakas)

Pulling directly from the Solr documentation for reference...

Tagging and excluding Filters

One can tag specific filters and exclude those filters when faceting. This is generally needed when doing multi-select faceting.

Consider the following example query with faceting:

q=mainquery&fq=status:public&fq=doctype:pdf&facet=on&facet.field=doctype

Because everything is already constrained by the filter doctype:pdf, the facet.field=doctype facet command is currently redundant and will return 0 counts for everything except doctype:pdf.

To implement a multi-select facet for doctype, a GUI may want to still display the other doctype values and their associated counts, as if the doctype:pdf constraint had not yet been applied. Example:

=== Document Type ===
  [ ] Word (42)
  [x] PDF  (96)
  [ ] Excel(11)
  [ ] HTML (63)

To return counts for doctype values that are currently not selected, tag filters that directly constrain doctype, and exclude those filters when faceting on doctype.

q=mainquery&fq=status:public&fq={!tag=dt}doctype:pdf&facet=on&facet.field={!ex=dt}doctype

This taggin concept is basically working as suggested:

Well, not as a general Search API / Facet API implementation a.t.m., but the following code simulates the suggested tagging and tagged-excluding on solr queries made by search_api with search_api_solr, on all involved facets for a certain search_api index. See it as a proof of (the tagging) concept, and/or change it (module name, limiting index name etc.) and use it to solve your (anyone) problem:

/*
 * Implementes _search_api_solr_query_alter().
 *
 * Add tags to fq fields and facet.field fields to exclude each facet's
 * filtering contraints from influencing its own facet item's result counts,
 * to always keep the result counts of each facet item independent of that
 * facet's filtering.
 *
 * See http://drupal.org/node/1446824# for background information.
 */
function ri_portfolio_search_api_solr_query_alter(array &$call_args, SearchApiQueryInterface $query) {
  // Only alter queries on the portfolio index.
  if ($query->getIndex()->machine_name != 'portfolio') return;

  // Go through all facet.field(s).
  foreach($call_args['params']['facet.field'] as $key => $facetfield) {
    // Come up with tag name for the facet.field.
    $tag = 'dtex'.$key;
    // Mark the tag form "exclude from counting" for this facet.
    $call_args['params']['facet.field'][$key] = "{!ex=$tag}" . $facetfield;
    // Tag the respective query field with the tag.
    _ri_portfolio_alter_solr_add_field_tag($call_args, $facetfield, $tag);
    
    // What the heck is done here? See example and explanation at:
    // Example from http://drupal.org/node/1446824#comment-5632892
    // q=mainquery&fq=status:public&fq=doctype:pdf&facet=on&facet.field=doctype
    // q=mainquery&fq=status:public&fq=         doctype:pdf&facet=on&facet.field=        doctype
    // q=mainquery&fq=status:public&fq={!tag=dt}doctype:pdf&facet=on&facet.field={!ex=dt}doctype
  }  
}


/**
 * Helper function to add a tag to a solr query field for search_api_solr query.
 */
function _ri_portfolio_alter_solr_add_field_tag(array &$call_args, $change_fieldname, $tag) {
  foreach($call_args['params']['fq'] as $key => $field) {
    $fieldname_end_pos = strpos($field, ':');
    $fieldname = substr($field, 0, $fieldname_end_pos);
    if ($fieldname == $change_fieldname) {
      $call_args['params']['fq'][$key] = "{!tag=$tag}" . $field;
    }
  }
}

So, how can we proceed to support this behaviour seemlessly from search_api_solr+search_api to facet_api ?
Chris from Facet API suggested to implement the solr-tagging in the backend (i.e. search_api + search_api_solr), and to offer and propagate a corresponding option for Facet API facets in the bridging code between the two modules.

Thomas (drunken monkey), do you agree?
Where do we attack within search_api(_solr) ?
How do we split work?
Let's discuss the search_api specifics here and coordinate the effort with Facet API over at #1446824: Support the multi-select facet feature of Solr where the associated counts are maintained as if no contraints have been applied, agreed?