Django rest framework recursive support (#110)

* add get_serializer_ref_name utility function
* implement RecursiveFieldInspector
* add option to allow non-existing reference in SchemaRef
* add examples and README
* Update changelog and docs

src/drf_yasg/openapi.py

@@ -466,7 +466,7 @@ class Schema(SwaggerDict):
 class _Ref(SwaggerDict):
     ref_name_re = re.compile(r"#/(?P<scope>.+)/(?P<name>[^/]+)$")
 
-    def __init__(self, resolver, name, scope, expected_type):
+    def __init__(self, resolver, name, scope, expected_type, ignore_unresolved=False):
         """Base class for all reference types. A reference object has only one property, ``$ref``, which must be a JSON
         reference to a valid object in the specification, e.g. ``#/definitions/Article`` to refer to an article model.
 
@@ -474,13 +474,15 @@ class _Ref(SwaggerDict):
         :param str name: referenced object name, e.g. "Article"
         :param str scope: reference scope, e.g. "definitions"
         :param type[.SwaggerDict] expected_type: the expected type that will be asserted on the object found in resolver
+        :param bool ignore_unresolved: allow the reference to be not defined in resolver
         """
         super(_Ref, self).__init__()
         assert not type(self) == _Ref, "do not instantiate _Ref directly"
         ref_name = "#/{scope}/{name}".format(scope=scope, name=name)
-        obj = resolver.get(name, scope)
-        assert isinstance(obj, expected_type), ref_name + " is a {actual}, not a {expected}" \
-            .format(actual=type(obj).__name__, expected=expected_type.__name__)
+        if not ignore_unresolved:
+            obj = resolver.get(name, scope)
+            assert isinstance(obj, expected_type), ref_name + " is a {actual}, not a {expected}" \
+                .format(actual=type(obj).__name__, expected=expected_type.__name__)
         self.ref = ref_name
 
     def resolve(self, resolver):
@@ -502,14 +504,15 @@ class _Ref(SwaggerDict):
 
 
 class SchemaRef(_Ref):
-    def __init__(self, resolver, schema_name):
+    def __init__(self, resolver, schema_name, ignore_unresolved=False):
         """Adds a reference to a named Schema defined in the ``#/definitions/`` object.
 
         :param .ReferenceResolver resolver: component resolver which must contain the definition
         :param str schema_name: schema name
+        :param bool ignore_unresolved: allow the reference to be not defined in resolver
         """
         assert SCHEMA_DEFINITIONS in resolver.scopes
-        super(SchemaRef, self).__init__(resolver, schema_name, SCHEMA_DEFINITIONS, Schema)
+        super(SchemaRef, self).__init__(resolver, schema_name, SCHEMA_DEFINITIONS, Schema, ignore_unresolved)
 
 
 Schema.OR_REF = (Schema, SchemaRef)