Solr currently cannot offer what I'm calling here a "context-sensitive" spelling suggestion. That is, if a user enters one or more words that have docFrequency > 0, but nevertheless are misspelled, then no suggestions are offered. Currently, Solr will always consider a word "correctly spelled" if it is in the index and/or dictionary, regardless of context. This issue & patch add support for context-sensitive spelling suggestions.
See SpellCheckCollatorTest.testContextSensitiveCollate() for a the typical use case for this functionality. This tests both using IndexBasedSepllChecker and DirectSolrSpellChecker.
Two new Spelling Parameters were added:
- spellcheck.alternativeTermCount - The count of suggestions to return for each query term existing in the index and/or dictionary. Presumably, users will want fewer suggestions for words with docFrequency>0. Also setting this value turns "on" context-sensitive spell suggestions.
- spellcheck.maxResultsForSuggest - The maximum number of hits the request can return in order to both generate spelling suggestions and set the "correctlySpelled" element to "false". For example, if this is set to 5 and the user's query returns 5 or fewer results, the spellchecker will report "correctlySpelled=false" and also offer suggestions (and collations if requested). Setting this greater than zero is useful for creating "did-you-mean" suggestions for queries that return a low number of hits.
I have also included a test using shards. See additions to DistributedSpellCheckComponentTest.
In Lucene, SpellChecker.java can already support this functionality (by passing a null IndexReader and field-name). The DirectSpellChecker, however, needs a minor enhancement. This gives the option to allow DirectSpellChecker to return suggestions for all query terms regardless of frequency.