Fixed #24938 -- Added PostgreSQL trigram support.

docs/ref/contrib/postgres/lookups.txt

@@ -2,6 +2,32 @@
 PostgreSQL specific lookups
 ===========================
 
+Trigram similarity
+==================
+
+.. fieldlookup:: trigram_similar
+
+.. versionadded:: 1.10
+
+The ``trigram_similar`` lookup allows you to perform trigram lookups,
+measuring the number of trigrams (three consecutive characters) shared, using a
+dedicated PostgreSQL extension. A trigram lookup is given an expression and
+returns results that have a similarity measurement greater than the current
+similarity threshold.
+
+To use it, add ``'django.contrib.postgres'`` in your :setting:`INSTALLED_APPS`
+and activate the `pg_trgm extension
+<http://www.postgresql.org/docs/current/interactive/pgtrgm.html>`_ on
+PostgreSQL. You can install the extension using the
+:class:`~django.contrib.postgres.operations.TrigramExtension` migration
+operation.
+
+The ``trigram_similar`` lookup can be used on
+:class:`~django.db.models.CharField` and :class:`~django.db.models.TextField`::
+
+    >>> City.objects.filter(name__trigram_similar="Middlesborough")
+    ['<City: Middlesbrough>']
+
 ``Unaccent``
 ============
 

docs/ref/contrib/postgres/operations.txt

@@ -27,6 +27,16 @@ the ``django.contrib.postgres.operations`` module.
     which will install the ``hstore`` extension and also immediately set up the
     connection to interpret hstore data.
 
+``TrigramExtension``
+====================
+
+.. class:: TrigramExtension()
+
+    .. versionadded:: 1.10
+
+    A subclass of :class:`~django.contrib.postgres.operations.CreateExtension`
+    that installs the ``pg_trgm`` extension.
+
 ``UnaccentExtension``
 =====================
 

docs/ref/contrib/postgres/search.txt

@@ -189,3 +189,58 @@ if it were an annotated ``SearchVector``::
     [<Entry: Cheese on Toast recipes>, <Entry: Pizza recipes>]
 
 .. _PostgreSQL documentation: http://www.postgresql.org/docs/current/static/textsearch-features.html#TEXTSEARCH-UPDATE-TRIGGERS
+
+Trigram similarity
+==================
+
+Another approach to searching is trigram similarity. A trigram is a group of
+three consecutive characters. In addition to the :lookup:`trigram_similar`
+lookup, you can use a couple of other expressions.
+
+To use them, you need to activate the `pg_trgm extension
+<http://www.postgresql.org/docs/current/interactive/pgtrgm.html>`_ on
+PostgreSQL. You can install it using the
+:class:`~django.contrib.postgres.operations.TrigramExtension` migration
+operation.
+
+``TrigramSimilarity``
+---------------------
+
+.. class:: TrigramSimilarity(expression, string, **extra)
+
+.. versionadded:: 1.10
+
+Accepts a field name or expression, and a string or expression. Returns the
+trigram similarity between the two arguments.
+
+Usage example::
+
+    >>> from django.contrib.postgres.search import TrigramSimilarity
+    >>> Author.objects.create(name='Katy Stevens')
+    >>> Author.objects.create(name='Stephen Keats')
+    >>> test = 'Katie Stephens'
+    >>> Author.objects.annotate(
+    ...     similarity=TrigramSimilarity('name', test),
+    ... ).filter(similarity__gt=0.3).order_by('-similarity')
+    [<Author: Katy Stephens>, <Author: Stephen Keats>]
+
+``TrigramDistance``
+-------------------
+
+.. class:: TrigramDistance(expression, string, **extra)
+
+.. versionadded:: 1.10
+
+Accepts a field name or expression, and a string or expression. Returns the
+trigram distance between the two arguments.
+
+Usage example::
+
+    >>> from django.contrib.postgres.search import TrigramDistance
+    >>> Author.objects.create(name='Katy Stevens')
+    >>> Author.objects.create(name='Stephen Keats')
+    >>> test = 'Katie Stephens'
+    >>> Author.objects.annotate(
+    ...     distance=TrigramDistance('name', test),
+    ... ).filter(distance__lte=0.7).order_by('distance')
+    [<Author: Katy Stephens>, <Author: Stephen Keats>]

docs/releases/1.10.txt

@@ -33,6 +33,10 @@ search engine. You can search across multiple fields in your relational
 database, combine the searches with other lookups, use different language
 configurations and weightings, and rank the results by relevance.
 
+It also now includes trigram support, using the :lookup:`trigram_similar`
+lookup, and the :class:`~django.contrib.postgres.search.TrigramSimilarity` and
+:class:`~django.contrib.postgres.search.TrigramDistance` expressions.
+
 Minor features
 --------------
 

docs/topics/db/search.txt

@@ -55,11 +55,12 @@ use :lookup:`unaccented comparison <unaccent>`::
 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 trigram comparison, which compares sequences of letters.
+would be to use a :lookup:`trigram_similar` comparison, which compares
+sequences of letters.
 
 For example::
 
-    >>> Author.objects.filter(name__unaccent__lower__trigram='Hélène')
+    >>> Author.objects.filter(name__unaccent__lower__trigram_similar='Hélène')
     [<Author: Helen Mirren>, <Actor: Hélène Joy>]
 
 Now we have a different problem - the longer name of "Helena Bonham Carter"