# Licensed to the Apache Software Foundation (ASF) under one or more # contributor license agreements. See the NOTICE file distributed with # this work for additional information regarding copyright ownership. # The ASF licenses this file to You under the Apache License, Version 2.0 # (the "License"); you may not use this file except in compliance with # the License. You may obtain a copy of the License at # # http://www.apache.org/licenses/LICENSE-2.0 # # Unless required by applicable law or agreed to in writing, software # distributed under the License is distributed on an "AS IS" BASIS, # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. # See the License for the specific language governing permissions and # limitations under the License. =head1 NAME Lucy::Docs::Cookbook::CustomQuery - Sample subclass of Query. =head1 ABSTRACT Explore Apache Lucy's support for custom query types by creating a "PrefixQuery" class to handle trailing wildcards. my $prefix_query = PrefixQuery->new( field => 'content', query_string => 'foo*', ); my $hits = $searcher->hits( query => $prefix_query ); ... =head1 Query, Compiler, and Matcher To add support for a new query type, we need three classes: a Query, a Compiler, and a Matcher. =over =item * PrefixQuery - a subclass of L, and the only class that client code will deal with directly. =item * PrefixCompiler - a subclass of L, whose primary role is to compile a PrefixQuery to a PrefixMatcher. =item * PrefixMatcher - a subclass of L, which does the heavy lifting: it applies the query to individual documents and assigns a score to each match. =back The PrefixQuery class on its own isn't enough because a Query object's role is limited to expressing an abstract specification for the search. A Query is basically nothing but metadata; execution is left to the Query's companion Compiler and Matcher. Here's a simplified sketch illustrating how a Searcher's hits() method ties together the three classes. sub hits { my ( $self, $query ) = @_; my $compiler = $query->make_compiler( searcher => $self ); my $matcher = $compiler->make_matcher( reader => $self->get_reader, need_score => 1, ); my @hits = $matcher->capture_hits; return \@hits; } =head2 PrefixQuery Our PrefixQuery class will have two attributes: a query string and a field name. package PrefixQuery; use base qw( Lucy::Search::Query ); use Carp; use Scalar::Util qw( blessed ); # Inside-out member vars and hand-rolled accessors. my %query_string; my %field; sub get_query_string { my $self = shift; return $query_string{$$self} } sub get_field { my $self = shift; return $field{$$self} } PrefixQuery's constructor collects and validates the attributes. sub new { my ( $class, %args ) = @_; my $query_string = delete $args{query_string}; my $field = delete $args{field}; my $self = $class->SUPER::new(%args); confess("'query_string' param is required") unless defined $query_string; confess("Invalid query_string: '$query_string'") unless $query_string =~ /\*\s*$/; confess("'field' param is required") unless defined $field; $query_string{$$self} = $query_string; $field{$$self} = $field; return $self; } Since this is an inside-out class, we'll need a destructor: sub DESTROY { my $self = shift; delete $query_string{$$self}; delete $field{$$self}; $self->SUPER::DESTROY; } The equals() method determines whether two Queries are logically equivalent: sub equals { my ( $self, $other ) = @_; return 0 unless blessed($other); return 0 unless $other->isa("PrefixQuery"); return 0 unless $field{$$self} eq $field{$$other}; return 0 unless $query_string{$$self} eq $query_string{$$other}; return 1; } The last thing we'll need is a make_compiler() factory method which kicks out a subclass of L. sub make_compiler { my $self = shift; return PrefixCompiler->new( @_, parent => $self ); } =head2 PrefixCompiler PrefixQuery's make_compiler() method will be called internally at search-time by objects which subclass L -- such as L. A Searcher is associated with a particular collection of documents. These documents may all reside in one index, as with IndexSearcher, or they may be spread out across multiple indexes on one or more machines, as with L. Searcher objects have access to certain statistical information about the collections they represent; for instance, a Searcher can tell you how many documents are in the collection... my $maximum_number_of_docs_in_collection = $searcher->doc_max; ... or how many documents a specific term appears in: my $term_appears_in_this_many_docs = $searcher->doc_freq( field => 'content', term => 'foo', ); Such information can be used by sophisticated Compiler implementations to assign more or less heft to individual queries or sub-queries. However, we're not going to bother with weighting for this demo; we'll just assign a fixed score of 1.0 to each matching document. We don't need to write a constructor, as it will suffice to inherit new() from Lucy::Search::Compiler. The only method we need to implement for PrefixCompiler is make_matcher(). package PrefixCompiler; use base qw( Lucy::Search::Compiler ); sub make_matcher { my ( $self, %args ) = @_; my $seg_reader = $args{reader}; # Retrieve low-level components LexiconReader and PostingListReader. my $lex_reader = $seg_reader->obtain("Lucy::Index::LexiconReader"); my $plist_reader = $seg_reader->obtain("Lucy::Index::PostingListReader"); # Acquire a Lexicon and seek it to our query string. my $substring = $self->get_parent->get_query_string; $substring =~ s/\*.\s*$//; my $field = $self->get_parent->get_field; my $lexicon = $lex_reader->lexicon( field => $field ); return unless $lexicon; $lexicon->seek($substring); # Accumulate PostingLists for each matching term. my @posting_lists; while ( defined( my $term = $lexicon->get_term ) ) { last unless $term =~ /^\Q$substring/; my $posting_list = $plist_reader->posting_list( field => $field, term => $term, ); if ($posting_list) { push @posting_lists, $posting_list; } last unless $lexicon->next; } return unless @posting_lists; return PrefixMatcher->new( posting_lists => \@posting_lists ); } PrefixCompiler gets access to a L object when make_matcher() gets called. From the SegReader and its sub-components L and L, we acquire a L, scan through the Lexicon's unique terms, and acquire a L for each term that matches our prefix. Each of these PostingList objects represents a set of documents which match the query. =head2 PrefixMatcher The Matcher subclass is the most involved. package PrefixMatcher; use base qw( Lucy::Search::Matcher ); # Inside-out member vars. my %doc_ids; my %tick; sub new { my ( $class, %args ) = @_; my $posting_lists = delete $args{posting_lists}; my $self = $class->SUPER::new(%args); # Cheesy but simple way of interleaving PostingList doc sets. my %all_doc_ids; for my $posting_list (@$posting_lists) { while ( my $doc_id = $posting_list->next ) { $all_doc_ids{$doc_id} = undef; } } my @doc_ids = sort { $a <=> $b } keys %all_doc_ids; $doc_ids{$$self} = \@doc_ids; # Track our position within the array of doc ids. $tick{$$self} = -1; return $self; } sub DESTROY { my $self = shift; delete $doc_ids{$$self}; delete $tick{$$self}; $self->SUPER::DESTROY; } The doc ids must be in order, or some will be ignored; hence the C above. In addition to the constructor and destructor, there are three methods that must be overridden. next() advances the Matcher to the next valid matching doc. sub next { my $self = shift; my $doc_ids = $doc_ids{$$self}; my $tick = ++$tick{$$self}; return 0 if $tick >= scalar @$doc_ids; return $doc_ids->[$tick]; } get_doc_id() returns the current document id, or 0 if the Matcher is exhausted. (L start at 1, so 0 is a sentinel.) sub get_doc_id { my $self = shift; my $tick = $tick{$$self}; my $doc_ids = $doc_ids{$$self}; return $tick < scalar @$doc_ids ? $doc_ids->[$tick] : 0; } score() conveys the relevance score of the current match. We'll just return a fixed score of 1.0: sub score { 1.0 } =head1 Usage To get a basic feel for PrefixQuery, insert the FlatQueryParser module described in L (which supports PrefixQuery) into the search.cgi sample app. my $parser = FlatQueryParser->new( schema => $searcher->get_schema ); my $query = $parser->parse($q); If you're planning on using PrefixQuery in earnest, though, you may want to change up analyzers to avoid stemming, because stemming -- another approach to prefix conflation -- is not perfectly compatible with prefix searches. # Polyanalyzer with no SnowballStemmer. my $analyzer = Lucy::Analysis::PolyAnalyzer->new( analyzers => [ Lucy::Analysis::RegexTokenizer->new, Lucy::Analysis::CaseFolder->new, ], ); =cut