Refs #28643 -- Added Repeat database function.

Thanks Tim Graham and Nick Pope for reviews.

django/db/backends/sqlite3/base.py

@@ -4,6 +4,7 @@ SQLite3 backend for the sqlite3 module in the standard library.
 import datetime
 import decimal
 import math
+import operator
 import re
 import warnings
 from sqlite3 import dbapi2 as Database
@@ -170,6 +171,7 @@ class DatabaseWrapper(BaseDatabaseWrapper):
         conn.create_function("django_format_dtdelta", 3, _sqlite_format_dtdelta)
         conn.create_function("django_power", 2, _sqlite_power)
         conn.create_function('LPAD', 3, _sqlite_lpad)
+        conn.create_function('REPEAT', 2, operator.mul)
         conn.create_function('RPAD', 3, _sqlite_rpad)
         conn.execute('PRAGMA foreign_keys = ON')
         return conn

django/db/models/functions/__init__.py

@@ -6,8 +6,8 @@ from .datetime import (
     TruncQuarter, TruncSecond, TruncTime, TruncWeek, TruncYear,
 )
 from .text import (
-    Chr, Concat, ConcatPair, Left, Length, Lower, LPad, LTrim, Ord, Replace,
-    Right, RPad, RTrim, StrIndex, Substr, Trim, Upper,
+    Chr, Concat, ConcatPair, Left, Length, Lower, LPad, LTrim, Ord, Repeat,
+    Replace, Right, RPad, RTrim, StrIndex, Substr, Trim, Upper,
 )
 from .window import (
     CumeDist, DenseRank, FirstValue, Lag, LastValue, Lead, NthValue, Ntile,
@@ -25,8 +25,8 @@ __all__ = [
     'TruncWeek', 'TruncYear',
     # text
     'Chr', 'Concat', 'ConcatPair', 'Left', 'Length', 'Lower', 'LPad', 'LTrim',
-    'Ord', 'Replace', 'Right', 'RPad', 'RTrim', 'StrIndex', 'Substr', 'Trim',
-    'Upper',
+    'Ord', 'Repeat', 'Replace', 'Right', 'RPad', 'RTrim', 'StrIndex', 'Substr',
+    'Trim', 'Upper',
     # window
     'CumeDist', 'DenseRank', 'FirstValue', 'Lag', 'LastValue', 'Lead',
     'NthValue', 'Ntile', 'PercentRank', 'Rank', 'RowNumber',

django/db/models/functions/text.py

@@ -152,6 +152,20 @@ class Ord(Transform):
         return super().as_sql(compiler, connection, function='UNICODE', **extra_context)
 
 
+class Repeat(BytesToCharFieldConversionMixin, Func):
+    function = 'REPEAT'
+
+    def __init__(self, expression, number, **extra):
+        if not hasattr(number, 'resolve_expression') and number < 0:
+            raise ValueError("'number' must be greater or equal to 0.")
+        super().__init__(expression, number, **extra)
+
+    def as_oracle(self, compiler, connection, **extra_context):
+        expression, number = self.source_expressions
+        rpad = RPad(expression, Length(expression) * number, expression)
+        return rpad.as_sql(compiler, connection, **extra_context)
+
+
 class Replace(Func):
     function = 'REPLACE'
 

docs/ref/models/database-functions.txt

@@ -855,6 +855,25 @@ Usage example::
     >>> print(author.name_code_point)
     77
 
+``Repeat``
+----------
+
+.. class:: Repeat(expression, number, **extra)
+
+.. versionadded:: 2.1
+
+Returns the value of the given text field or expression repeated ``number``
+times.
+
+Usage example::
+
+    >>> from django.db.models.functions import Repeat
+    >>> Author.objects.create(name='John', alias='j')
+    >>> Author.objects.update(name=Repeat('name', 3))
+    1
+    >>> print(Author.objects.get(alias='j').name)
+    JohnJohnJohn
+
 ``Replace``
 -----------
 

docs/releases/2.1.txt

@@ -210,6 +210,7 @@ Models
   :class:`~django.db.models.functions.LPad`,
   :class:`~django.db.models.functions.LTrim`,
   :class:`~django.db.models.functions.Ord`,
+  :class:`~django.db.models.functions.Repeat`,
   :class:`~django.db.models.functions.Replace`,
   :class:`~django.db.models.functions.Right`,
   :class:`~django.db.models.functions.RPad`,

tests/db_functions/test_repeat.py

@@ -0,0 +1,24 @@
+from django.db.models import CharField, Value
+from django.db.models.functions import Length, Repeat
+from django.test import TestCase
+
+from .models import Author
+
+
+class RepeatTests(TestCase):
+    def test_basic(self):
+        Author.objects.create(name='John', alias='xyz')
+        tests = (
+            (Repeat('name', 0), ''),
+            (Repeat('name', 2), 'JohnJohn'),
+            (Repeat('name', Length('alias'), output_field=CharField()), 'JohnJohnJohn'),
+            (Repeat(Value('x'), 3, output_field=CharField()), 'xxx'),
+        )
+        for function, repeated_text in tests:
+            with self.subTest(function=function):
+                authors = Author.objects.annotate(repeated_text=function)
+                self.assertQuerysetEqual(authors, [repeated_text], lambda a: a.repeated_text, ordered=False)
+
+    def test_negative_number(self):
+        with self.assertRaisesMessage(ValueError, "'number' must be greater or equal to 0."):
+            Repeat('name', -1)