Fixed #19299 -- Fixed Nullification of Foreign Keys To CharFields

Thanks tunixman for the report and Baptiste Mispelon and
Shai Berger for reviews.

2 files changed, 9 insertions, 5 deletions

diff --git a/django/db/models/fields/related.py b/django/db/models/fields/related.py
index f490066516..98ec36b656 100644
--- a/django/db/models/fields/related.py
+++ b/django/db/models/fields/related.py
@@ -1692,11 +1692,12 @@ class ForeignKey(ForeignObject):
         return field_default
 
     def get_db_prep_save(self, value, connection):
-        if value == '' or value is None:
+        if value is None or (value == '' and
+                             (not self.related_field.empty_strings_allowed or
+                              connection.features.interprets_empty_strings_as_nulls)):
             return None
         else:
-            return self.related_field.get_db_prep_save(value,
-                connection=connection)
+            return self.related_field.get_db_prep_save(value, connection=connection)
 
     def value_to_string(self, obj):
         if not obj:
diff --git a/django/db/models/query.py b/django/db/models/query.py
index 6051b9f859..725c04caca 100644
--- a/django/db/models/query.py
+++ b/django/db/models/query.py
@@ -1416,9 +1416,12 @@ def get_cached_row(row, index_start, using, klass_info, offset=0,
     klass, field_names, field_count, related_fields, reverse_related_fields, pk_idx = klass_info
 
     fields = row[index_start:index_start + field_count]
-    # If the pk column is None (or the Oracle equivalent ''), then the related
+    # If the pk column is None (or the equivalent '' in the case the
+    # connection interprets empty strings as nulls), then the related
     # object must be non-existent - set the relation to None.
-    if fields[pk_idx] is None or fields[pk_idx] == '':
+    if (fields[pk_idx] is None or
+        (connections[using].features.interprets_empty_strings_as_nulls and
+         fields[pk_idx] == '')):
         obj = None
     elif field_names:
         fields = list(fields)