Union types in connections: 'UnionOptions' object has no attribute 'model'

从 SQLAlchemyConnectionField.wrap_resolve() 及其 model 属性入手，然后使用由 SQLAlchemyObjectType 类组成的 Graphene union 重现报告中的查询。跟踪 union 的 _meta 数据如何传递到 connection resolver；完成这项工作需要定义如何处理具有多个底层模型的 connection，同时避免报告中的 UnionOptions.model 错误。

Are there plans to add support for union types in connections? I have a setup like the following:

class Cat(SQLAlchemyObjectType):
    class Meta:
        model = CatModel
        interfaces = (relay.Node,)

class Dog(SQLAlchemyObjectType):
    class Meta:
        model = DogModel
        interfaces = (relay.Node,)

class Animal(graphene.Union):
    class Meta:
        types = (Cat, Dog)

class Query(graphene.ObjectType):
    animals = SQLAlchemyConnectionField(Animal)

and when I try to query for animals, I get the error: 'UnionOptions' object has no attribute 'model'.

I traced this back to the self.model call in wrap_resolve():

    def wrap_resolve(self, parent_resolver):
        return partial(
            self.connection_resolver,
            parent_resolver,
            get_nullable_type(self.type),
            self.model,
        )

    def model(self):
        return get_nullable_type(self.type)._meta.node._meta.model

When the type is a union, get_nullable_type(self.type)._meta.node._meta returns the union class (in this case Animal). In order to get a model from that, we have to drill down into its types (e.g. get_nullable_type(self.type)._meta.node._meta.types[0]._meta.model) but because it's a union, it will contain multiple models.
Any ideas how to get around this?