Raise better error if type is missing from schema

graphene/types/tests/test_typemap.py

@@ -1,3 +1,4 @@
+import pytest
 
 from graphql.type import (GraphQLArgument, GraphQLEnumType, GraphQLEnumValue,
                           GraphQLField, GraphQLInputObjectField,
@@ -13,7 +14,7 @@ from ..inputobjecttype import InputObjectType
 from ..interface import Interface
 from ..objecttype import ObjectType
 from ..scalars import String, Int
-from ..typemap import TypeMap
+from ..typemap import TypeMap, resolve_type
 
 
 def test_enum():
@@ -232,3 +233,22 @@ def test_objecttype_with_possible_types():
     assert graphql_type.is_type_of
     assert graphql_type.is_type_of({}, None) is True
     assert graphql_type.is_type_of(MyObjectType(), None) is False
+
+
+def test_resolve_type_with_missing_type():
+    class MyObjectType(ObjectType):
+        foo_bar = String()
+
+    class MyOtherObjectType(ObjectType):
+        fizz_buzz = String()
+
+    def resolve_type_func(root, info):
+        return MyOtherObjectType
+
+    typemap = TypeMap([MyObjectType])
+    with pytest.raises(AssertionError) as excinfo:
+        resolve_type(
+            resolve_type_func, typemap, 'MyOtherObjectType', {}, {}
+        )
+
+    assert 'MyOtherObjectTyp' in str(excinfo.value)

graphene/types/typemap.py

@@ -46,7 +46,10 @@ def resolve_type(resolve_type_func, map, type_name, root, info):
 
     if inspect.isclass(_type) and issubclass(_type, ObjectType):
         graphql_type = map.get(_type._meta.name)
-        assert graphql_type and graphql_type.graphene_type == _type, (
+        assert graphql_type, "Can't find type {} in schema".format(
+            _type._meta.name
+        )
+        assert graphql_type.graphene_type == _type, (
             'The type {} does not match with the associated graphene type {}.'
         ).format(_type, graphql_type.graphene_type)
         return graphql_type