[1.8.x] Fixed #24486 -- Fixed error with datetime and DurationField arithmetic

Backport of ff2aa4019259947734c4791d6afc9f5405d901d0 from master

2 files changed, 15 insertions, 5 deletions

diff --git a/django/db/models/expressions.py b/django/db/models/expressions.py
index 41ce6ba187..0e328ccf55 100644
--- a/django/db/models/expressions.py
+++ b/django/db/models/expressions.py
@@ -398,9 +398,10 @@ class DurationExpression(Expression):
                 output = side.output_field
             except FieldError:
                 pass
-            if output.get_internal_type() == 'DurationField':
-                sql, params = compiler.compile(side)
-                return connection.ops.format_for_duration_arithmetic(sql), params
+            else:
+                if output.get_internal_type() == 'DurationField':
+                    sql, params = compiler.compile(side)
+                    return connection.ops.format_for_duration_arithmetic(sql), params
         return compiler.compile(side)
 
     def as_sql(self, compiler, connection):
diff --git a/tests/expressions/tests.py b/tests/expressions/tests.py
index 59fd063103..6297d68095 100644
--- a/tests/expressions/tests.py
+++ b/tests/expressions/tests.py
@@ -617,8 +617,8 @@ class ExpressionOperatorTests(TestCase):
 class FTimeDeltaTests(TestCase):
 
     def setUp(self):
-        sday = datetime.date(2010, 6, 25)
-        stime = datetime.datetime(2010, 6, 25, 12, 15, 30, 747000)
+        self.sday = sday = datetime.date(2010, 6, 25)
+        self.stime = stime = datetime.datetime(2010, 6, 25, 12, 15, 30, 747000)
         midnight = datetime.time(0)
 
         delta0 = datetime.timedelta(0)
@@ -821,6 +821,15 @@ class FTimeDeltaTests(TestCase):
             Experiment.objects.filter(estimated_time__lt=F('end') - F('start'))]
         self.assertEqual(over_estimate, ['e4'])
 
+    def test_duration_with_datetime(self):
+        # Exclude e1 which has very high precision so we can test this on all
+        # backends regardless of whether or not it supports
+        # microsecond_precision.
+        over_estimate = Experiment.objects.exclude(name='e1').filter(
+            completed__gt=self.stime + F('estimated_time'),
+        ).order_by('name')
+        self.assertQuerysetEqual(over_estimate, ['e3', 'e4'], lambda e: e.name)
+
 
 class ValueTests(TestCase):
     def test_update_TimeField_using_Value(self):