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You may want to use multi-select faceting to constrain the results shown for one facet but still want to see the values and counts of other facets that are not matched by your constraint. To solve this you often had to use two queries. Since Solr 1.4 you can tag and exclude filters and save the additional query.
“I want to use memcached for my ActiveRecord models and am looking for recommendations on how to do this”
Of the available gems the memcached gem can be recommended. The memcached gem wraps the libmemcached client which is written in pure C. This implementation gives it a performance advantage over the memcached_client gem.