Fixed #4136 -- Made ModelForm save empty values for nullable CharFields as NULL.

Previously, empty values were saved as strings.

django/db/models/base.py

@@ -13,8 +13,8 @@ from django.core.exceptions import (
     ObjectDoesNotExist, ValidationError,
 )
 from django.db import (
-    DEFAULT_DB_ALIAS, DJANGO_VERSION_PICKLE_KEY, DatabaseError, connections,
-    router, transaction,
+    DEFAULT_DB_ALIAS, DJANGO_VERSION_PICKLE_KEY, DatabaseError, connection,
+    connections, router, transaction,
 )
 from django.db.models import signals
 from django.db.models.constants import LOOKUP_SEP
@@ -1087,7 +1087,9 @@ class Model(six.with_metaclass(ModelBase)):
             for field_name in unique_check:
                 f = self._meta.get_field(field_name)
                 lookup_value = getattr(self, f.attname)
-                if lookup_value is None:
+                # TODO: Handle multiple backends with different feature flags.
+                if (lookup_value is None or
+                        (lookup_value == '' and connection.features.interprets_empty_strings_as_nulls)):
                     # no value, skip the lookup
                     continue
                 if f.primary_key and not self._state.adding:

django/db/models/fields/__init__.py

@@ -1086,6 +1086,9 @@ class CharField(Field):
         # will be validated twice. This is considered acceptable since we want
         # the value in the form field (to pass into widget for example).
         defaults = {'max_length': self.max_length}
+        # TODO: Handle multiple backends with different feature flags.
+        if self.null and not connection.features.interprets_empty_strings_as_nulls:
+            defaults['empty_value'] = None
         defaults.update(kwargs)
         return super(CharField, self).formfield(**defaults)
 

django/forms/fields.py

@@ -214,10 +214,11 @@ class Field(object):
 
 
 class CharField(Field):
-    def __init__(self, max_length=None, min_length=None, strip=True, *args, **kwargs):
+    def __init__(self, max_length=None, min_length=None, strip=True, empty_value='', *args, **kwargs):
         self.max_length = max_length
         self.min_length = min_length
         self.strip = strip
+        self.empty_value = empty_value
         super(CharField, self).__init__(*args, **kwargs)
         if min_length is not None:
             self.validators.append(validators.MinLengthValidator(int(min_length)))
@@ -227,7 +228,7 @@ class CharField(Field):
     def to_python(self, value):
         "Returns a Unicode object."
         if value in self.empty_values:
-            return ''
+            return self.empty_value
         value = force_text(value)
         if self.strip:
             value = value.strip()