Fixed #27834 -- Added StrIndex database function.

django/db/models/functions/__init__.py

 from .base import (
     Cast, Coalesce, Concat, ConcatPair, Greatest, Least, Length, Lower, Now,
-    Substr, Upper,
+    StrIndex, Substr, Upper,
 )
 from .datetime import (
     Extract, ExtractDay, ExtractHour, ExtractMinute, ExtractMonth,
@@ -12,7 +12,7 @@ from .datetime import (
 __all__ = [
     # base
     'Cast', 'Coalesce', 'Concat', 'ConcatPair', 'Greatest', 'Least', 'Length',
-    'Lower', 'Now', 'Substr', 'Upper',
+    'Lower', 'Now', 'StrIndex', 'Substr', 'Upper',
     # datetime
     'Extract', 'ExtractDay', 'ExtractHour', 'ExtractMinute', 'ExtractMonth',
     'ExtractSecond', 'ExtractWeek', 'ExtractWeekDay', 'ExtractYear',

django/db/models/functions/base.py

@@ -187,6 +187,29 @@ class Now(Func):
         return self.as_sql(compiler, connection, template='STATEMENT_TIMESTAMP()')
 
 
+class StrIndex(Func):
+    """
+    Return a positive integer corresponding to the 1-indexed position of the
+    first occurrence of a substring inside another string, or 0 if the
+    substring is not found.
+    """
+    function = 'INSTR'
+    arity = 2
+
+    def __init__(self, expression, substring, **extra):
+        """
+        expression: the name of a field, or an expression returning a string
+        substring: a string to find inside expression
+        """
+        if not hasattr(substring, 'resolve_expression'):
+            substring = Value(substring)
+        expressions = [expression, substring]
+        super().__init__(*expressions, output_field=fields.IntegerField(), **extra)
+
+    def as_postgresql(self, compiler, connection):
+        return super().as_sql(compiler, connection, function='STRPOS')
+
+
 class Substr(Func):
     function = 'SUBSTRING'
 

docs/ref/models/database-functions.txt

@@ -237,6 +237,38 @@ Usage example::
     ``Now()`` uses ``STATEMENT_TIMESTAMP`` instead. If you need the transaction
     timestamp, use :class:`django.contrib.postgres.functions.TransactionNow`.
 
+``StrIndex``
+============
+
+.. class:: StrIndex(expression, substring, **extra)
+
+.. versionadded:: 2.0
+
+Returns a positive integer corresponding to the 1-indexed position of the
+first occurrence of ``substring`` inside another string, or 0 if the substring
+is not found.
+
+Usage example::
+
+    >>> from django.db.models.functions import StrIndex
+    >>> Author.objects.create(name='Margaret Smith')
+    >>> Author.objects.create(name='Smith, Margaret')
+    >>> Author.objects.create(name='Margaret Jackson')
+    >>> authors = Author.objects.annotate(
+    ...     smith_index=StrIndex('name', 'Smith')).order_by('smith_index')
+    >>> authors.first().smith_index
+    0
+    >>> authors = Author.objects.annotate(
+    ...    smith_index=StrIndex('name', 'Smith')).filter(smith_index__gt=0)
+    <QuerySet [<Author: Margaret Smith>, <Author: Smith, Margaret>]>
+
+.. warning::
+
+    In MySQL, a database table's :ref:`collation<mysql-collation>` determines
+    whether string comparisons (such as the ``expression`` and ``substring`` of
+    this function) are case-sensitive. Comparisons are case-insensitive by
+    default.
+
 ``Substr``
 ==========
 

docs/releases/2.0.txt

@@ -170,7 +170,8 @@ Migrations
 Models
 ~~~~~~
 
-* ...
+* The new :class:`~django.db.models.functions.StrIndex` database function
+  finds the starting index of a string inside another string.
 
 Requests and Responses
 ~~~~~~~~~~~~~~~~~~~~~~

tests/db_functions/test_strindex.py

+from django.db.models.functions import StrIndex
+from django.test import TestCase
+from django.utils import timezone
+
+from .models import Article, Author
+
+
+class StrIndexTests(TestCase):
+    def test_annotate_charfield(self):
+        Author.objects.create(name='George. R. R. Martin')
+        Author.objects.create(name='J. R. R. Tolkien')
+        Author.objects.create(name='Terry Pratchett')
+        authors = Author.objects.annotate(fullstop=StrIndex('name', 'R.'))
+        self.assertQuerysetEqual(authors.order_by('name'), [9, 4, 0], lambda a: a.fullstop)
+
+    def test_annotate_textfield(self):
+        Article.objects.create(
+            title='How to Django',
+            text='Lorem ipsum dolor sit amet.',
+            written=timezone.now(),
+        )
+        Article.objects.create(
+            title='How to Tango',
+            text="Won't find anything here.",
+            written=timezone.now(),
+        )
+        articles = Article.objects.annotate(ipsum_index=StrIndex('text', 'ipsum'))
+        self.assertQuerysetEqual(articles.order_by('title'), [7, 0], lambda a: a.ipsum_index)
+
+    def test_order_by(self):
+        Author.objects.create(name='Terry Pratchett')
+        Author.objects.create(name='J. R. R. Tolkien')
+        Author.objects.create(name='George. R. R. Martin')
+        self.assertQuerysetEqual(
+            Author.objects.order_by(StrIndex('name', 'R.').asc()), [
+                'Terry Pratchett',
+                'J. R. R. Tolkien',
+                'George. R. R. Martin',
+            ],
+            lambda a: a.name
+        )
+        self.assertQuerysetEqual(
+            Author.objects.order_by(StrIndex('name', 'R.').desc()), [
+                'George. R. R. Martin',
+                'J. R. R. Tolkien',
+                'Terry Pratchett',
+            ],
+            lambda a: a.name
+        )
+
+    def test_unicode_values(self):
+        Author.objects.create(name='ツリー')
+        Author.objects.create(name='皇帝')
+        Author.objects.create(name='皇帝 ツリー')
+        authors = Author.objects.annotate(sb=StrIndex('name', 'リ'))
+        self.assertQuerysetEqual(authors.order_by('name'), [2, 0, 5], lambda a: a.sb)
+
+    def test_filtering(self):
+        Author.objects.create(name='George. R. R. Martin')
+        Author.objects.create(name='Terry Pratchett')
+        self.assertQuerysetEqual(
+            Author.objects.annotate(middle_name=StrIndex('name', 'R.')).filter(middle_name__gt=0),
+            ['George. R. R. Martin'],
+            lambda a: a.name
+        )