Updated docs reflecting static resolvers

(And a working example of is_type_of)

Author: syrusakbary

docs/types/objecttypes.rst

@@ -55,6 +55,12 @@ otherwise, the ``resolve_{field_name}`` within the ``ObjectType``.
 By default a resolver will take the ``args``, ``context`` and ``info``
 arguments.
 
+NOTE: The class resolvers in a ``ObjectType`` are treated as ``staticmethod``s
+always, so the first argument in the resolver: ``self`` (or ``root``) doesn't
+need to be an actual instance of the ``ObjectType``. In the case this happens, please
+overwrite the ``is_type_of`` method.
+
+
 Quick example
 ~~~~~~~~~~~~~
 
@@ -90,6 +96,53 @@ A field could also specify a custom resolver outside the class:
         reverse = graphene.String(word=graphene.String(), resolver=reverse)
 
 
+Is Type Of
+----------
+
+An ``ObjectType`` could be resolved within a object that is not an instance of this
+``ObjectType``. That means that the resolver of a ``Field`` could return any object.
+
+Let's see an example:
+
+.. code:: python
+    import graphene
+
+    class Ship:
+        def __init__(self, name):
+            self.name = name
+
+    class ShipType(graphene.ObjectType):
+        name = graphene.String(description="Ship name", required=True)
+
+        @resolve_only_args
+        def resolve_name(self):
+            # Here self will be the Ship instance returned in resolve_ship
+            return self.name
+
+    class Query(graphene.ObjectType):
+        ship = graphene.Field(ShipNode)
+
+        def resolve_ship(self, context, args, info):
+            return Ship(name='xwing')
+
+    schema = graphene.Schema(query=Query)
+
+
+In this example, we are returning a ``Ship`` which is not an instance of ``ShipType``.
+If we execute a query on the ship, we would see this error:
+`"Expected value of type \"ShipType\" but got: instance."`
+
+That's happening because GraphQL have no idea what type ``Ship`` is. For solving this,
+we only have to add a ``is_type_of`` method in ``ShipType``
+
+.. code:: python
+
+    class ShipType(graphene.ObjectType):
+        @classmethod
+        def is_type_of(cls, root, context, info):
+            return isinstance(root, (Ship, ShipType))
+
+
 Instances as data containers
 ----------------------------