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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
Official home page for acts_as_xapian:
http://github.com/frabcus/acts_as_xapian/wikis
a. Introduction to acts_as_xapian
=========
Xapian 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 or Ultrasphinx.
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. Run it on a cron job, or similar.
Xapian 1.0.5 and associated Ruby bindings are required. In Debian or Ubuntu
install the packages libxapian15 and libxapian-ruby1.8.
Email [email protected] with patches.
Installation===
Retrieve the plugin directly from the git version control system by running
this command within your Rails app.
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 – maybe this works in acts_as_solr, but I never found
out how.
- Spelling correction – “did you mean?” built in and just works.
- Multiple models – acts_as_xapian searches multiple models 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.
- 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
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
=======
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 after 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
- prefix is the part to use in search queries that goes before the :
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.
Alternatively,
- :instead_index, a field which refers to another model that should be reindexed
instead of this one.
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
2. Generate a database migration to create the ActsAsXapianJob model:
script/generate acts_as_xapian rake db:migrate3. 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.
4. 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)
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
- :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, 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
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.
For more details about anything, see source code in lib/acts_as_xapian.rb -
please though 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.
The official home page and repository for acts_as_xapian are here:
http://github.com/frabcus/acts_as_xapian/wikis