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====================================
Model Data to Support Keyword Search
====================================
.. default-domain:: mongodb
.. note::
Keyword search is *not* the same as text search or full text
search, and does not provide stemming or other text-processing
features. See the :ref:`limit-keyword-indexes` section for more
information.
In 2.4, MongoDB provides a text search feature. See
:doc:`/core/index-text` for more information.
If your application needs to perform queries on the content of a field
that holds text you can perform exact matches on the text or use
:query:`$regex` to use regular expression pattern matches. However,
for many operations on text, these methods do not satisfy application
requirements.
This pattern describes one method for supporting keyword search using
MongoDB to support application search functionality, that uses
keywords stored in an array in the same document as the text
field. Combined with a :ref:`multi-key index <index-type-multikey>`,
this pattern can support application's keyword search operations.
Pattern
-------
To add structures to your document to support keyword-based queries,
create an array field in your documents and add the keywords as
strings in the array. You can then create a :ref:`multi-key index
<index-type-multi-key>` on the array and create queries that select
values from the array.
.. example::
Given a collection of library volumes that you want to provide
topic-based search. For each volume, you add the array ``topics``,
and you add as many keywords as needed for a given volume.
For the ``Moby-Dick`` volume you might have the following document:
.. code-block:: javascript
{ title : "Moby-Dick" ,
author : "Herman Melville" ,
published : 1851 ,
ISBN : 0451526996 ,
topics : [ "whaling" , "allegory" , "revenge" , "American" ,
"novel" , "nautical" , "voyage" , "Cape Cod" ]
}
You then create a multi-key index on the ``topics`` array:
.. code-block:: javascript
db.volumes.ensureIndex( { topics: 1 } )
The multi-key index creates separate index entries for each keyword in
the ``topics`` array. For example the index contains one entry for
``whaling`` and another for ``allegory``.
You then query based on the keywords. For example:
.. code-block:: javascript
db.volumes.findOne( { topics : "voyage" }, { title: 1 } )
.. note:: An array with a large number of elements, such as one with
several hundreds or thousands of keywords will incur greater
indexing costs on insertion.
.. _limit-keyword-indexes:
Limitations of Keyword Indexes
------------------------------
MongoDB can support keyword searches using specific data models and
:ref:`multi-key indexes <index-type-multikey>`; however, these keyword
indexes are not sufficient or comparable to full-text products in the
following respects:
- *Stemming*. Keyword queries in MongoDB can not parse keywords for
root or related words.
- *Synonyms*. Keyword-based search features must provide support for
synonym or related queries in the application layer.
- *Ranking*. The keyword look ups described in this document do not
provide a way to weight results.
- *Asynchronous Indexing*. MongoDB builds indexes synchronously, which
means that the indexes used for keyword indexes are always current
and can operate in real-time. However, asynchronous bulk indexes
may be more efficient for some kinds of content and workloads.
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