Ferret

Filtering Search Results

We’ve already mentioned Filters in our discussion of ConstantScoreQuery and FilteredQuery. Filters are used to apply extra constraints to a result set. For example, we want to restrict our search to documents that were created during the last month. We have two options: add a RangeQuery clause to our query, or apply a RangeFilter. The main advantage of using a Filter over a Query is that no score is taken into account, so a Filter can be a lot faster. To add to that, Filters cache their results so that subsequent uses of the Filter perform even better again. All caching is done against an instance of an IndexReader, so a new cache needs to be built each time a Filter is used against a different IndexReader.

Filters also make it easy to apply constraints to user input queries. Filters are best used when applying commonly used constraints to a user’s query, such as restricting a search of a blog to only today’s postings or only to postings marked for publication.

There are only two standard Filters that come with Ferret:

• RangeFilter

• QueryFilter

Using the RangeFilter

RangeFilter takes the same parameters as RangeQuery as described in the RangeQuery” section earlier in this chapter. Basically, you need to supply a :field and an upper and/or lower limit for that field. For example, if you want to restrict a search to products that are priced at $50.00 or more and less than $100.00, we would build the filter like this:

price_filter = RangeFilter.new(:price, :>=