fixes fields behaviour when null value is assigned

rest_framework/fields.py

@@ -429,6 +429,11 @@ class ModelField(WritableField):
             "type": self.model_field.get_internal_type()
         }
 
+    def validate(self, value):
+        super(ModelField, self).validate(value)
+        if value is None and not self.model_field.null:
+            raise ValidationError(self.error_messages['invalid'])
+
 
 ##### Typed Fields #####
 
@@ -474,10 +479,7 @@ class CharField(WritableField):
             self.validators.append(validators.MaxLengthValidator(max_length))
 
     def from_native(self, value):
-        if isinstance(value, six.string_types):
-            return value
-
-        if value is None:
+        if value in validators.EMPTY_VALUES:
             return ''
 
         return smart_text(value)

rest_framework/relations.py

@@ -33,7 +33,7 @@ class RelatedField(WritableField):
     many_widget = widgets.SelectMultiple
     form_field_class = forms.ChoiceField
     many_form_field_class = forms.MultipleChoiceField
-    null_values = (None, '', 'None')
+    null_values = (None, '', 'None', [], (), {})
 
     cache_choices = False
     empty_label = None
@@ -182,7 +182,7 @@ class RelatedField(WritableField):
         if value in self.null_values:
             if self.required:
                 raise ValidationError(self.error_messages['required'])
-            into[(self.source or field_name)] = None
+            into[(self.source or field_name)] = [] if self.many else None
         elif self.many:
             into[(self.source or field_name)] = [self.from_native(item) for item in value]
         else: