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

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

AUTHORS

@@ -646,6 +646,7 @@ answer newbie questions, and generally made Django that much better:
     Rick Wagner <rwagner@physics.ucsd.edu>
     Gavin Wahl <gavinwahl@gmail.com>
     wam-djangobug@wamber.net
+    Albert Wang <aywang31@gmail.com>
     Wang Chun <wangchun@exoweb.net>
     Filip Wasilewski <filip.wasilewski@gmail.com>
     Dan Watson <http://danwatson.net/>

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:

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)

tests/model_fields/models.py

@@ -72,12 +72,21 @@ class BooleanModel(models.Model):
     string = models.CharField(max_length=10, default='abc')
 
 
+class PrimaryKeyCharModel(models.Model):
+    string = models.CharField(max_length=10, primary_key=True)
+
+
 class FksToBooleans(models.Model):
     """Model wih FKs to models with {Null,}BooleanField's, #15040"""
     bf = models.ForeignKey(BooleanModel)
     nbf = models.ForeignKey(NullBooleanModel)
 
 
+class FkToChar(models.Model):
+    """Model with FK to a model with a CharField primary key, #19299"""
+    out = models.ForeignKey(PrimaryKeyCharModel)
+
+
 class RenamedField(models.Model):
     modelname = models.IntegerField(name="fieldname", choices=((1, 'One'),))
 

tests/model_fields/tests.py

@@ -22,8 +22,8 @@ from django.utils.functional import lazy
 
 from .models import (
     Foo, Bar, Whiz, BigD, BigS, BigInt, Post, NullBooleanModel,
-    BooleanModel, DataModel, Document, RenamedField,
-    VerboseNameField, FksToBooleans)
+    BooleanModel, PrimaryKeyCharModel, DataModel, Document, RenamedField,
+    VerboseNameField, FksToBooleans, FkToChar)
 
 
 class BasicFieldTests(test.TestCase):
@@ -146,6 +146,17 @@ class ForeignKeyTests(test.TestCase):
         b = Bar.objects.create(b="bcd")
         self.assertEqual(b.a, a)
 
+    @test.skipIfDBFeature('interprets_empty_strings_as_nulls')
+    def test_empty_string_fk(self):
+        """
+        Test that foreign key values to empty strings don't get converted
+        to None (#19299)
+        """
+        char_model_empty = PrimaryKeyCharModel.objects.create(string='')
+        fk_model_empty = FkToChar.objects.create(out=char_model_empty)
+        fk_model_empty = FkToChar.objects.select_related('out').get(id=fk_model_empty.pk)
+        self.assertEqual(fk_model_empty.out, char_model_empty)
+
 
 class DateTimeFieldTests(unittest.TestCase):
     def test_datetimefield_to_python_usecs(self):