[1.5.x] Fixed #20091 -- Oracle null promotion for empty strings

Backpatch of e17fa9e877e84e93b699c2bd13ea48dbbb86e451

1 files changed, 8 insertions, 1 deletions

diff --git a/django/db/models/sql/query.py b/django/db/models/sql/query.py
index e7c8d6caaf..7368fed24c 100644
--- a/django/db/models/sql/query.py
+++ b/django/db/models/sql/query.py
@@ -1106,7 +1106,14 @@ class Query(object):
             # If value is a query expression, evaluate it
             value = SQLEvaluator(value, self, reuse=can_reuse)
             having_clause = value.contains_aggregate
-
+        # For Oracle '' is equivalent to null. The check needs to be done
+        # at this stage because join promotion can't be done at compiler
+        # stage. Using DEFAULT_DB_ALIAS isn't nice, but it is the best we
+        # can do here. Similar thing is done in is_nullable(), too.
+        if (connections[DEFAULT_DB_ALIAS].features.interprets_empty_strings_as_nulls and
+                lookup_type == 'exact' and value == ''):
+            value = True
+            lookup_type = 'isnull'
         for alias, aggregate in self.aggregates.items():
             if alias in (parts[0], LOOKUP_SEP.join(parts)):
                 entry = self.where_class()