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

Backport of 6c95b134e9b2d5641c123551c080305e90e6a89d from master

1 files changed, 9 insertions, 6 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):