The facet.prefix parameter limits the terms on which to facet to those *starting with the given string prefix*.
It appears that you are looking to find it based on sequence of characters appears in the middle of the string, this goes against the *facet.prefix* definition. Have you considered copying the /my_facet_field/(string field) into /my_search_field/ (text field; analyzed to suit your search needs); With that approach you can search in /my_search_field/ and facet on /my_facet_field/ -- Sent from: http://lucene.472066.n3.nabble.com/Solr-User-f472068.html
