NAME¶
Lucy::Docs::Cookbook::CustomQueryParser - Sample subclass of QueryParser.
ABSTRACT¶
Implement a custom search query language using a subclass of
Lucy::Search::QueryParser.
The language¶
At first, our query language will support only simple term queries and phrases
delimited by double quotes. For simplicity's sake, it will not support
parenthetical groupings, boolean operators, or prepended plus/minus. The
results for all subqueries will be unioned together -- i.e. joined using an OR
-- which is usually the best approach for small-to-medium-sized document
collections.
Later, we'll add support for trailing wildcards.
Single-field parser¶
Our initial parser implentation will generate queries against a single fixed
field, "content", and it will analyze text using a fixed choice of
English PolyAnalyzer. We won't subclass Lucy::Search::QueryParser just yet.
package FlatQueryParser;
use Lucy::Search::TermQuery;
use Lucy::Search::PhraseQuery;
use Lucy::Search::ORQuery;
use Carp;
sub new {
my $analyzer = Lucy::Analysis::PolyAnalyzer->new(
language => 'en',
);
return bless {
field => 'content',
analyzer => $analyzer,
}, __PACKAGE__;
}
Some private helper subs for creating TermQuery and PhraseQuery objects will
help keep the size of our main
parse() subroutine down:
sub _make_term_query {
my ( $self, $term ) = @_;
return Lucy::Search::TermQuery->new(
field => $self->{field},
term => $term,
);
}
sub _make_phrase_query {
my ( $self, $terms ) = @_;
return Lucy::Search::PhraseQuery->new(
field => $self->{field},
terms => $terms,
);
}
Our private
_tokenize() method treats double-quote delimited material as
a single token and splits on whitespace everywhere else.
sub _tokenize {
my ( $self, $query_string ) = @_;
my @tokens;
while ( length $query_string ) {
if ( $query_string =~ s/^\s+// ) {
next; # skip whitespace
}
elsif ( $query_string =~ s/^("[^"]*(?:"|$))// ) {
push @tokens, $1; # double-quoted phrase
}
else {
$query_string =~ s/(\S+)//;
push @tokens, $1; # single word
}
}
return \@tokens;
}
The main parsing routine creates an array of tokens by calling
_tokenize(), runs the tokens through through the PolyAnalyzer, creates
TermQuery or PhraseQuery objects according to how many tokens emerge from the
PolyAnalyzer's
split() method, and adds each of the sub-queries to the
primary ORQuery.
sub parse {
my ( $self, $query_string ) = @_;
my $tokens = $self->_tokenize($query_string);
my $analyzer = $self->{analyzer};
my $or_query = Lucy::Search::ORQuery->new;
for my $token (@$tokens) {
if ( $token =~ s/^"// ) {
$token =~ s/"$//;
my $terms = $analyzer->split($token);
my $query = $self->_make_phrase_query($terms);
$or_query->add_child($phrase_query);
}
else {
my $terms = $analyzer->split($token);
if ( @$terms == 1 ) {
my $query = $self->_make_term_query( $terms->[0] );
$or_query->add_child($query);
}
elsif ( @$terms > 1 ) {
my $query = $self->_make_phrase_query($terms);
$or_query->add_child($query);
}
}
}
return $or_query;
}
Multi-field parser¶
Most often, the end user will want their search query to match not only a single
'content' field, but also 'title' and so on. To make that happen, we have to
turn queries such as this...
foo AND NOT bar
... into the logical equivalent of this:
(title:foo OR content:foo) AND NOT (title:bar OR content:bar)
Rather than continue with our own from-scratch parser class and write the
routines to accomplish that expansion, we're now going to subclass
Lucy::Search::QueryParser and take advantage of some of its existing methods.
Our first parser implementation had the "content" field name and the
choice of English PolyAnalyzer hard-coded for simplicity, but we don't need to
do that once we subclass Lucy::Search::QueryParser. QueryParser's constructor
-- which we will inherit, allowing us to eliminate our own constructor --
requires a Schema which conveys field and Analyzer information, so we can just
defer to that.
package FlatQueryParser;
use base qw( Lucy::Search::QueryParser );
use Lucy::Search::TermQuery;
use Lucy::Search::PhraseQuery;
use Lucy::Search::ORQuery;
use PrefixQuery;
use Carp;
# Inherit new()
We're also going to jettison our
_make_term_query() and
_make_phrase_query() helper subs and chop our
parse() subroutine
way down. Our revised
parse() routine will generate
Lucy::Search::LeafQuery objects instead of TermQueries and PhraseQueries:
sub parse {
my ( $self, $query_string ) = @_;
my $tokens = $self->_tokenize($query_string);
my $or_query = Lucy::Search::ORQuery->new;
for my $token (@$tokens) {
my $leaf_query = Lucy::Search::LeafQuery->new( text => $token );
$or_query->add_child($leaf_query);
}
return $self->expand($or_query);
}
The magic happens in QueryParser's
expand() method, which walks the
ORQuery object we supply to it looking for LeafQuery objects, and calls
expand_leaf() for each one it finds.
expand_leaf() performs
field-specific analysis, decides whether each query should be a TermQuery or a
PhraseQuery, and if multiple fields are required, creates an ORQuery which
mults out e.g. "foo" into "(title:foo OR content:foo)".
Extending the query language¶
To add support for trailing wildcards to our query language, we need to override
expand_leaf() to accommodate PrefixQuery, while deferring to the parent
class implementation on TermQuery and PhraseQuery.
sub expand_leaf {
my ( $self, $leaf_query ) = @_;
my $text = $leaf_query->get_text;
if ( $text =~ /\*$/ ) {
my $or_query = Lucy::Search::ORQuery->new;
for my $field ( @{ $self->get_fields } ) {
my $prefix_query = PrefixQuery->new(
field => $field,
query_string => $text,
);
$or_query->add_child($prefix_query);
}
return $or_query;
}
else {
return $self->SUPER::expand_leaf($leaf_query);
}
}
Ordinarily, those asterisks would have been stripped when running tokens through
the PolyAnalyzer -- query strings containing "foo*" would produce
TermQueries for the term "foo". Our override intercepts tokens with
trailing asterisks and processes them as PrefixQueries before
"SUPER::expand_leaf" can discard them, so that a search for
"foo*" can match "food", "foosball", and so on.
Usage¶
Insert our custom parser into the search.cgi sample app to get a feel for how it
behaves:
my $parser = FlatQueryParser->new( schema => $searcher->get_schema );
my $query = $parser->parse( decode( 'UTF-8', $cgi->param('q') || '' ) );
my $hits = $searcher->hits(
query => $query,
offset => $offset,
num_wanted => $page_size,
);
...