Don't validate anything except if present.

Closes #81

jsonschema.py

@@ -208,7 +208,13 @@ class ValidatorMixin(object):
             _schema = self.schema
 
         with self.resolver.in_scope(_schema.get("id", "")):
-            for k, v in iteritems(_schema):
+            ref = _schema.get("$ref")
+            if ref is not None:
+                validators = [("$ref", ref)]
+            else:
+                validators = iteritems(_schema)
+
+            for k, v in validators:
                 validator_attr = "validate_%s" % (k.lstrip("$"),)
                 validator = getattr(self, validator_attr, None)
 

tests.py

@@ -25,9 +25,9 @@ except ImportError:
     pypy_version_info = None
 
 from jsonschema import (
-    PY3, SchemaError, UnknownType, ValidationError, ErrorTree,
-    Draft3Validator, Draft4Validator, FormatChecker, draft3_format_checker,
-    draft4_format_checker, RefResolver, validate, FormatError
+    PY3, FormatError, SchemaError, UnknownType, ValidationError, ErrorTree,
+    Draft3Validator, Draft4Validator, FormatChecker, RefResolver,
+    ValidatorMixin, draft3_format_checker, draft4_format_checker, validate,
 )
 
 
@@ -613,6 +613,18 @@ class TestValidate(unittest.TestCase):
             chk_schema.assert_called_once_with({})
 
 
+class TestValidatorMixin(unittest.TestCase):
+    def test_if_ref_is_present_then_the_schema_is_replaced(self):
+        class Validator(ValidatorMixin):
+            validate_ref = mock.Mock(return_value=[])
+            validate_type = mock.Mock(return_value=[])
+
+        Validator({"$ref" : "foo", "type" : "quux"}).validate(1)
+
+        self.assertTrue(Validator.validate_ref.called)
+        self.assertFalse(Validator.validate_type.called)
+
+
 class TestRefResolver(unittest.TestCase):
     def setUp(self):
         self.base_uri = ""