Limit blank string -> None to just be on relational fields

rest_framework/fields.py

@@ -110,7 +110,6 @@ class Field(object):
     default_validators = []
     default_empty_html = empty
     initial = None
-    coerce_blank_to_null = True
 
     def __init__(self, read_only=False, write_only=False,
                  required=None, default=empty, initial=empty, source=None,
@@ -248,9 +247,6 @@ class Field(object):
                 self.fail('required')
             return self.get_default()
 
-        if data == '' and self.coerce_blank_to_null:
-            data = None
-
         if data is None:
             if not self.allow_null:
                 self.fail('null')

rest_framework/relations.py

 from rest_framework.compat import smart_text, urlparse
-from rest_framework.fields import Field
+from rest_framework.fields import empty, Field
 from rest_framework.reverse import reverse
 from django.core.exceptions import ObjectDoesNotExist, ImproperlyConfigured
 from django.core.urlresolvers import resolve, get_script_prefix, NoReverseMatch, Resolver404
@@ -31,6 +31,12 @@ class RelatedField(Field):
             )
         return super(RelatedField, cls).__new__(cls, *args, **kwargs)
 
+    def run_validation(self, data=empty):
+        # We force empty strings to None values for relational fields.
+        if data == '':
+            data = None
+        return super(RelatedField, self).run_validation(data)
+
     def get_queryset(self):
         queryset = self.queryset
         if isinstance(queryset, QuerySet):