fix(graphene): add a custom metaclass factory for interface types

Interface types, such as `Question`, `Answer`, and `Task` have subclasses,
and connections using those types may return a mix of specific object types.
Examples are form -> questions, which may return different question types.

For these interface types, the `graphene` internals require access to
a `FooType.Meta._meta.registry`  property, to access the type registry.
Somehow, the graphene metaclass system does not automatically build this
up correctly, so we have to help a little bit to make it work

caluma/caluma_core/filters.py

@@ -200,6 +200,38 @@ def _should_include_filter(filt):
     return filter_coll()
 
 
+def InterfaceMetaFactory():
+    """
+    Metaclass factory for interface types.
+
+    Build a meta class suitable for the schema type classes that represent
+    "interface" types (those that have concrete subclasses, but could be mixed
+    in a connection type, such as Question, Answer, and Task).
+
+    Usage:
+
+    >>> class Foo(Node, graphene.Interface):
+    >>>     ...
+    >>>     Meta = InterfaceMetaFactory()
+    """
+
+    class _meta(graphene.types.interface.InterfaceOptions):
+        @classmethod
+        # This is kinda useless but required as graphene tries to freeze()
+        # it's meta class objects
+        def freeze(cls):
+            cls._frozen = True
+
+        # This is what we're actually "fixing": On non-Interface types,
+        # this somehow works (or isn't needed), but here, if _meta.registry
+        # is not set, the whole schema construction fails
+        registry = get_global_registry()
+
+    # We need a new type (= class) each time it's called, because reuse
+    # triggers some weird errors
+    return type("Meta", (), {"_meta": _meta})
+
+
 def CollectionFilterSetFactory(filterset_class, orderset_class=None):
     """
     Build single-filter filterset classes.

caluma/caluma_form/schema.py

@@ -8,6 +8,7 @@
     CollectionFilterSetFactory,
     DjangoFilterConnectionField,
     DjangoFilterInterfaceConnectionField,
+    InterfaceMetaFactory,
 )
 from ..caluma_core.mutation import Mutation, UserDefinedPrimaryKeyMixin
 from ..caluma_core.relay import extract_global_id
@@ -159,6 +160,8 @@ def get_queryset(cls, queryset, info):
     def resolve_type(cls, instance, info):
         return resolve_question(instance)
 
+    Meta = InterfaceMetaFactory()
+
 
 class Option(FormDjangoObjectType):
     meta = generic.GenericScalar()
@@ -810,6 +813,8 @@ class Answer(Node, graphene.Interface):
     def resolve_type(cls, instance, info):
         return resolve_answer(instance)
 
+    Meta = InterfaceMetaFactory()
+
 
 class AnswerQuerysetMixin(object):
     """Mixin to combine all different answer types into one queryset."""

caluma/caluma_workflow/schema.py

@@ -10,6 +10,7 @@
     CollectionFilterSetFactory,
     DjangoFilterConnectionField,
     DjangoFilterInterfaceConnectionField,
+    InterfaceMetaFactory,
 )
 from ..caluma_core.mutation import Mutation, UserDefinedPrimaryKeyMixin
 from ..caluma_core.types import (
@@ -97,6 +98,8 @@ def resolve_type(cls, instance, info):
 
         return TASK_TYPE[instance.type]
 
+    Meta = InterfaceMetaFactory()
+
 
 class TaskConnection(CountableConnectionBase):
     class Meta:

caluma/tests/__snapshots__/test_schema.ambr

@@ -831,6 +831,21 @@
   type DjangoDebug {
     """Executed SQL queries for this API query."""
     sql: [DjangoDebugSQL]
+
+    """Raise exceptions for this API query."""
+    exceptions: [DjangoDebugException]
+  }
+
+  """Represents a single exception raised."""
+  type DjangoDebugException {
+    """The class of the exception"""
+    excType: String!
+
+    """The message of the exception"""
+    message: String!
+
+    """The stack trace"""
+    stack: String!
   }
 
   """Represents a single database query made to a Django managed DB."""