wbzyl-acts_as_xapian

Do patch this file if there is documentation missing / wrong. It's called README.txt and is in git, using Textile formatting. The wiki page is just copied from the README.txt file.

Contents

• a. Introduction to acts_as_xapian
• b. Installation
• c. Comparison to acts_as_solr (as on 24 April 2008)
• d. Documentation - indexing
• e. Documentation - querying
• f. Configuration
• g. Performance
• h. Support

a. Introduction to acts_as_xapian

"Xapian":http://www.xapian.org is a full text search engine library which has Ruby bindings. acts_as_xapian adds support for it to Rails. It is an alternative to acts_as_solr, acts_as_ferret, Ultrasphinx, acts_as_indexed, acts_as_searchable or acts_as_tsearch.

acts_as_xapian is deployed in production on these websites.

• "WhatDoTheyKnow":http://www.whatdotheyknow.com
• "MindBites":http://www.mindbites.com

The section "c. Comparison to acts_as_solr" below will give you an idea of acts_as_xapian's features.

acts_as_xapian was started by Francis Irving in May 2008 for search and email alerts in WhatDoTheyKnow, and so was supported by "mySociety":http://www.mysociety.org and initially paid for by the "JRSST Charitable Trust":http://www.jrrt.org.uk/jrsstct.htm

b. Installation

Retrieve the plugin directly from the git version control system by running this command within your Rails app.

git clone git://github.com/frabcus/acts_as_xapian.git vendor/plugins/acts_as_xapian

Xapian 1.0.5 and associated Ruby bindings are also required.

Debian or Ubuntu - install the packages libxapian15 and libxapian-ruby1.8.

Mac OSX - follow the instructions for installing from source on the "Installing Xapian":http://xapian.org/docs/install.html page - you need the Xapian library and bindings (you don't need Omega).

There is no Ruby Gem for Xapian, it would be great if you could make one!

c. Comparison to acts_as_solr (as on 24 April 2008)

• Offline indexing only mode - which is a minus if you want changes immediately reflected in the search index, and a plus if you were going to have to implement your own offline indexing anyway.

• Collapsing - the equivalent of SQL's "group by". You can specify a field to collapse on, and only the most relevant result from each value of that field is returned. Along with a count of how many there are in total. acts_as_solr doesn't have this.

• No highlighting - Xapian can't return you text highlighted with a search query. You can try and make do with TextHelper::highlight (combined with words_to_highlight below). I found the highlighting in acts_as_solr didn't really understand the query anyway.

• Date range searching - this exists in acts_as_solr, but I found it wasn't documented well enough, and was hard to get working.

• Spelling correction - "did you mean?" built in and just works.

• Similar documents - acts_as_xapian has a simple command to find other models that are like a specified model.

• Multiple models - acts_as_xapian searches multiple types of model if you like, returning them mixed up together by relevancy. This is like multi_solr_search, only it is the default mode of operation and is properly supported.

• No daemons - However, if you have more than one web server, you'll need to work out how to use "Xapian's remote backend":http://xapian.org/docs/remote.html.

• One layer - full-powered Xapian is called directly from the Ruby, without Solr getting in the way whenever you want to use a new feature from Lucene.

• No Java - an advantage if you're more used to working in the rest of the open source world. acts_as_xapian, it's pure Ruby and C++.

• Xapian's awesome email list - the kids over at "xapian-discuss":http://lists.xapian.org/mailman/listinfo/xapian-discuss are super helpful. Useful if you need to extend and improve acts_as_xapian. The Ruby bindings are mature and well maintained as part of Xapian.

d. Documentation - indexing

Xapian is an offline indexing search library - only one process can have the Xapian database open for writing at once, and others that try meanwhile are unceremoniously kicked out. For this reason, acts_as_xapian does not support immediate writing to the database when your models change.

Instead, there is a ActsAsXapianJob model which stores which models need updating or deleting in the search index. A rake task 'xapian:update_index' then performs the updates since last change. You can run it on a cron job, or similar.

Here's how to add indexing to your Rails app:

1. Put acts_as_xapian in your models that need search indexing. e.g.

   acts_as_xapian :texts => [ :name, :short_name ], :values => [ [ :created_at, 0, "created_at", :date ] ], :terms => [ [ :variety, 'V', "variety" ] ]

Options must include:

• :texts, an array of fields for indexing with full text search. e.g. :texts => [ :title, :body ]

• :values, things which have a range of values for sorting, or for collapsing. Specify an array quadruple of [ field, identifier, prefix, type ] where ** identifier is an arbitary numeric identifier for use in the Xapian database ** prefix is the part to use in search queries that goes before the : ** type can be any of :string, :number or :date

e.g. :values => [ [ :created_at, 0, "created_at", :date ], [ :size, 1, "size", :string ] ]

• :terms, things which come with a prefix (before a :) in search queries. Specify an array triple of [ field, char, prefix ] where ** char is an arbitary single upper case char used in the Xapian database, just pick any single uppercase character, but use a different one for each prefix. ** prefix is the part to use in search queries that goes before the : For example, if you were making Google and indexing to be able to later do a query like "site:www.whatdotheyknow.com", then the prefix would be "site".

e.g. :terms => [ [ :variety, 'V', "variety" ] ]

A 'field' is a symbol referring to either an attribute or a function which returns the text, date or number to index. Both 'identifier' and 'char' must be the same for the same prefix in different models.

Options may include:

• :eager_load, added as an :include clause when looking up search results in database
• :if, either an attribute or a function which if returns false means the object isn't indexed

1. Generate a database migration to create the ActsAsXapianJob model:

   script/generate acts_as_xapian rake db:migrate

2. Call 'rake xapian:rebuild_index models="ModelName1 ModelName2"' to build the index the first time (you must specify all your indexed models). It's put in a development/test/production dir in acts_as_xapian/xapiandbs. See f. Configuration below if you want to change this.

3. Then from a cron job or a daemon, or by hand regularly!, call 'rake xapian:update_index'

e. Documentation - querying

Testing indexing

If you just want to test indexing is working, you'll find this rake task useful (it has more options, see tasks/xapian.rake)

rake xapian:query models="PublicBody User" query="moo"

Performing a query

To perform a query from code call ActsAsXapian::Search.new. This takes in turn:

• model_classes - list of models to search, e.g. [PublicBody, InfoRequestEvent]
• query_string - Google like syntax, see below

And then a hash of options:

• :offset - Offset of first result (default 0)
• :limit - Number of results per page
• :sort_by_prefix - Optionally, prefix of value to sort by, otherwise sort by relevance
• :sort_by_ascending - Default true (documents with higher values better/earlier), set to false for descending sort
• :collapse_by_prefix - Optionally, prefix of value to collapse by (i.e. only return most relevant result from group)

Google like query syntax is as described in "Xapian::QueryParser Syntax":http://www.xapian.org/docs/queryparser.html Queries can include prefix:value parts, according to what you indexed in the acts_as_xapian part above. You can also say things like model:InfoRequestEvent to constrain by model in more complex ways than the :model parameter, or modelid:InfoRequestEvent-100 to only find one specific object.

Returns an ActsAsXapian::Search object. Useful methods are:

• description - a techy one, to check how the query has been parsed
• matches_estimated - a guesstimate at the total number of hits
• spelling_correction - the corrected query string if there is a correction, otherwise nil
• words_to_highlight - list of words for you to highlight, perhaps with TextHelper::highlight
• results - an array of hashes each containing: ** :model - your Rails model, this is what you most want! ** :weight - relevancy measure ** :percent - the weight as a %, 0 meaning the item did not match the query at all ** :collapse_count - number of results with the same prefix, if you specified collapse_by_prefix

Finding similar models

To find models that are similar to a given set of models call ActsAsXapian::Similar.new. This takes:

• model_classes - list of model classes to return models from within
• models - list of models that you want to find related ones to

Returns an ActsAsXapian::Similar object. Has all methods from ActsAsXapian::Search above, except for words_to_highlight. In addition has:

• important_terms - the terms extracted from the input models, that were used to search for output You need the results methods to get the similar models.

f. Configuration

If you want to customise the configuration of acts_as_xapian, it will look for a file called 'xapian.yml' under RAILS_ROOT/config. As is familiar from the format of the database.yml file, separate :development, :test and :production sections are expected.

The following options are available:

• base_db_path - specifies the directory, relative to RAILS_ROOT, in which acts_as_xapian stores its search index databases. Default is the directory xapiandbs within the acts_as_xapian directory.

g. Performance

On development sites, acts_as_xapian automatically logs the time taken to do searches. The time displayed is for the Xapian parts of the query; the Rails database model lookups will be logged separately by ActiveRecord. Example:

Xapian query (0.00029s) Search: hello

To enable this, and other performance logging, on a production site, temporarily add this to the end of your config/environment.rb

ActiveRecord::Base.logger = Logger.new(STDOUT)

h. Support

Please ask any questions on the "acts_as_xapian Google Group":http://groups.google.com/group/acts_as_xapian

The official home page and repository for acts_as_xapian are the "acts_as_xapian github page":http://github.com/frabcus/acts_as_xapian/wikis

For more details about anything, see source code in lib/acts_as_xapian.rb

Merging source instructions "Using git for collaboration" here: http://www.kernel.org/pub/software/scm/git/docs/gittutorial.html