mirror of
https://github.com/django/django.git
synced 2024-12-24 10:05:46 +00:00
131 lines
5.6 KiB
Plaintext
131 lines
5.6 KiB
Plaintext
======
|
|
Search
|
|
======
|
|
|
|
A common task for web applications is to search some data in the database with
|
|
user input. In a simple case, this could be filtering a list of objects by a
|
|
category. A more complex use case might require searching with weighting,
|
|
categorization, highlighting, multiple languages, and so on. This document
|
|
explains some of the possible use cases and the tools you can use.
|
|
|
|
We'll refer to the same models used in :doc:`/topics/db/queries`.
|
|
|
|
Use Cases
|
|
=========
|
|
|
|
Standard textual queries
|
|
------------------------
|
|
|
|
Text-based fields have a selection of simple matching operations. For example,
|
|
you may wish to allow lookup up an author like so::
|
|
|
|
>>> Author.objects.filter(name__contains='Terry')
|
|
[<Author: Terry Gilliam>, <Author: Terry Jones>]
|
|
|
|
This is a very fragile solution as it requires the user to know an exact
|
|
substring of the author's name. A better approach could be a case-insensitive
|
|
match (:lookup:`icontains`), but this is only marginally better.
|
|
|
|
A database's more advanced comparison functions
|
|
-----------------------------------------------
|
|
|
|
If you're using PostgreSQL, Django provides :doc:`a selection of database
|
|
specific tools </ref/contrib/postgres/search>` to allow you to leverage more
|
|
complex querying options. Other databases have different selections of tools,
|
|
possibly via plugins or user-defined functions. Django doesn't include any
|
|
support for them at this time. We'll use some examples from PostgreSQL to
|
|
demonstrate the kind of functionality databases may have.
|
|
|
|
.. admonition:: Searching in other databases
|
|
|
|
All of the searching tools provided by :mod:`django.contrib.postgres` are
|
|
constructed entirely on public APIs such as :doc:`custom lookups
|
|
</ref/models/lookups>` and :doc:`database functions
|
|
</ref/models/database-functions>`. Depending on your database, you should
|
|
be able to construct queries to allow similar APIs. If there are specific
|
|
things which cannot be achieved this way, please open a ticket.
|
|
|
|
In the above example, we determined that a case insensitive lookup would be
|
|
more useful. When dealing with non-English names, a further improvement is to
|
|
use :lookup:`unaccented comparison <unaccent>`::
|
|
|
|
>>> Author.objects.filter(name__unaccent__icontains='Helen')
|
|
[<Author: Helen Mirren>, <Author: Helena Bonham Carter>, <Author: Hélène Joy>]
|
|
|
|
This shows another issue, where we are matching against a different spelling of
|
|
the name. In this case we have an asymmetry though - a search for ``Helen``
|
|
will pick up ``Helena`` or ``Hélène``, but not the reverse. Another option
|
|
would be to use a :lookup:`trigram_similar` comparison, which compares
|
|
sequences of letters.
|
|
|
|
For example::
|
|
|
|
>>> Author.objects.filter(name__unaccent__lower__trigram_similar='Hélène')
|
|
[<Author: Helen Mirren>, <Author: Hélène Joy>]
|
|
|
|
Now we have a different problem - the longer name of "Helena Bonham Carter"
|
|
doesn't show up as it is much longer. Trigram searches consider all
|
|
combinations of three letters, and compares how many appear in both search and
|
|
source strings. For the longer name, there are more combinations which appear
|
|
in the source string so it is no longer considered a close match.
|
|
|
|
The correct choice of comparison functions here depends on your particular data
|
|
set, for example the language(s) used and the type of text being searched. All
|
|
of the examples we've seen are on short strings where the user is likely to
|
|
enter something close (by varying definitions) to the source data.
|
|
|
|
Document-based search
|
|
---------------------
|
|
|
|
Simple database operations are too simple an approach when you start
|
|
considering large blocks of text. Whereas the examples above can be thought of
|
|
as operations on a string of characters, full text search looks at the actual
|
|
words. Depending on the system used, it's likely to use some of the following
|
|
ideas:
|
|
|
|
- Ignoring "stop words" such as "a", "the", "and".
|
|
- Stemming words, so that "pony" and "ponies" are considered similar.
|
|
- Weighting words based on different criteria such as how frequently they
|
|
appear in the text, or the importance of the fields, such as the title or
|
|
keywords, that they appear in.
|
|
|
|
There are many alternatives for using searching software, some of the most
|
|
prominent are Elastic_ and Solr_. These are full document-based search
|
|
solutions. To use them with data from Django models, you'll need a layer which
|
|
translates your data into a textual document, including back-references to the
|
|
database ids. When a search using the engine returns a certain document, you
|
|
can then look it up in the database. There are a variety of third-party
|
|
libraries which are designed to help with this process.
|
|
|
|
.. _Elastic: https://www.elastic.co/
|
|
.. _Solr: http://lucene.apache.org/solr/
|
|
|
|
PostgreSQL support
|
|
~~~~~~~~~~~~~~~~~~
|
|
|
|
PostgreSQL has its own full text search implementation built-in. While not as
|
|
powerful as some other search engines, it has the advantage of being inside
|
|
your database and so can easily be combined with other relational queries such
|
|
as categorization.
|
|
|
|
The :mod:`django.contrib.postgres` module provides some helpers to make these
|
|
queries. For example, a simple query might be to select all the blog entries
|
|
which mention "cheese"::
|
|
|
|
>>> Entry.objects.filter(body_text__search='cheese')
|
|
[<Entry: Cheese on Toast recipes>, <Entry: Pizza recipes>]
|
|
|
|
You can also filter on a combination of fields and on related models::
|
|
|
|
>>> Entry.objects.annotate(
|
|
... search=SearchVector('blog__tagline', 'body_text'),
|
|
... ).filter(search='cheese')
|
|
[
|
|
<Entry: Cheese on Toast recipes>,
|
|
<Entry: Pizza Recipes>,
|
|
<Entry: Dairy farming in Argentina>,
|
|
]
|
|
|
|
See the ``contrib.postgres`` :doc:`/ref/contrib/postgres/search` document for
|
|
complete details.
|