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

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

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):