[1.8.x] Fixed #25517 -- Made Concat function idempotent on SQLite.

Backport of 6c95b134e9b2d5641c123551c080305e90e6a89d from master

3 files changed, 28 insertions, 7 deletions

diff --git a/django/db/models/functions.py b/django/db/models/functions.py
index 610ecb6985..531f9a882f 100644
--- a/django/db/models/functions.py
+++ b/django/db/models/functions.py
@@ -41,10 +41,10 @@ class ConcatPair(Func):
         super(ConcatPair, self).__init__(left, right, **extra)
 
     def as_sqlite(self, compiler, connection):
-        self.arg_joiner = ' || '
-        self.template = '%(expressions)s'
-        self.coalesce()
-        return super(ConcatPair, self).as_sql(compiler, connection)
+        coalesced = self.coalesce()
+        coalesced.arg_joiner = ' || '
+        coalesced.template = '%(expressions)s'
+        return super(ConcatPair, coalesced).as_sql(compiler, connection)
 
     def as_mysql(self, compiler, connection):
         self.coalesce()
@@ -52,9 +52,12 @@ class ConcatPair(Func):
 
     def coalesce(self):
         # null on either side results in null for expression, wrap with coalesce
+        c = self.copy()
         expressions = [
-            Coalesce(expression, Value('')) for expression in self.get_source_expressions()]
-        self.set_source_expressions(expressions)
+            Coalesce(expression, Value('')) for expression in c.get_source_expressions()
+        ]
+        c.set_source_expressions(expressions)
+        return c
 
 
 class Concat(Func):
diff --git a/docs/releases/1.8.6.txt b/docs/releases/1.8.6.txt
index 44380cded5..178d714f01 100644
--- a/docs/releases/1.8.6.txt
+++ b/docs/releases/1.8.6.txt
@@ -23,3 +23,5 @@ Bugfixes
   have their reverse relations disabled (:ticket:`25545`).
 
 * Allowed filtering over a ``RawSQL`` annotation (:ticket:`25506`).
+
+* Made the ``Concat`` database function idempotent on SQLite (:ticket:`25517`).
diff --git a/tests/db_functions/tests.py b/tests/db_functions/tests.py
index f07e8770f0..5fd3b74fbd 100644
--- a/tests/db_functions/tests.py
+++ b/tests/db_functions/tests.py
@@ -1,8 +1,11 @@
 from __future__ import unicode_literals
 
+from unittest import skipUnless
+
+from django.db import connection
 from django.db.models import CharField, TextField, Value as V
 from django.db.models.functions import (
-    Coalesce, Concat, Length, Lower, Substr, Upper,
+    Coalesce, Concat, ConcatPair, Length, Lower, Substr, Upper,
 )
 from django.test import TestCase
 from django.utils import six, timezone
@@ -154,6 +157,19 @@ class FunctionTests(TestCase):
         expected = article.title + ' - ' + article.text
         self.assertEqual(expected.upper(), article.title_text)
 
+    @skipUnless(connection.vendor == 'sqlite', "sqlite specific implementation detail.")
+    def test_concat_coalesce_idempotent(self):
+        pair = ConcatPair(V('a'), V('b'))
+        # Check nodes counts
+        self.assertEqual(len(list(pair.flatten())), 3)
+        self.assertEqual(len(list(pair.coalesce().flatten())), 7)  # + 2 Coalesce + 2 Value()
+        self.assertEqual(len(list(pair.flatten())), 3)
+
+    def test_concat_sql_generation_idempotency(self):
+        qs = Article.objects.annotate(description=Concat('title', V(': '), 'summary'))
+        # Multiple compilations should not alter the generated query.
+        self.assertEqual(str(qs.query), str(qs.all().query))
+
     def test_lower(self):
         Author.objects.create(name='John Smith', alias='smithj')
         Author.objects.create(name='Rhonda')