Nested inputs in mutations

I'm failing to convert nested inputs into sqlalchemy models in mutations. Here's my example:

Let's say I want to create a quiz. For that I have the following code:

```
'''
GraphQL Models
'''
class Quiz(SQLAlchemyObjectType):
'''
GraphQL representation of a Quiz
'''
class Meta:
model = QuizModel

class Question(SQLAlchemyObjectType):
'''
GraphQL representation of a Question
'''
class Meta:
model = QuestionModel

'''
Inputs
'''
class QuestionInput(graphene.InputObjectType):
text = graphene.String()
media = graphene.String()

class QuizInput(graphene.InputObjectType):
name = graphene.String()
creator_id = graphene.Int()
description = graphene.String()
media = graphene.String()
id = graphene.Int()
debugging_questions = graphene.InputField(graphene.List(QuestionInput))
questions = graphene.InputField(graphene.List(QuestionInput))

'''
Mutation
'''
class CreateQuiz(graphene.Mutation):
class Arguments:
quiz = QuizInput(required=True)

quiz = graphene.Field(lambda: Quiz)

def mutate(self, info, **kwargs):
quiz_attributes = dict(kwargs['quiz'])
if quiz_attributes.get('id'):
quiz = db_session.query(QuizModel).filter((QuizModel.id == quiz_attributes.get('id'))).one()
quiz_attributes.pop('id')
for key, value in quiz_attributes.items():
setattr(quiz, key, value)
else:
quiz = QuizModel(**quiz_attributes)

db_session.add(quiz)
db_session.commit()

return CreateQuiz(quiz=quiz)

```

My GraphQL query is the following:
```
mutation creatingQuiz($quiz: QuizInput!) {
createQuiz(quiz: $quiz) {
quiz {
id,
name,
description,
media,
creatorId,
questions {
id,
text,
media,
}
}
}
}
```

Note the relation between the global variables and the response:
Example A - returns a response, obviously doesn't add any quizzes because it uses `debuggingQuestions`.
```
Global variables:
{
"quiz": {
"id": 136,
"name": "fake name",
"description": "simple desc",
"creatorId": 1,
"media": "img.jpg",
"debuggingQuestions": [{
"media": "media",
"text": "text"
}]
}
}

Response:
{
"data": {
"createQuiz": {
"quiz": {
"id": "136",
"name": "fake name",
"description": "simple desc",
"media": "img.jpg",
"creatorId": 1,
"questions": []
}
}
}
}
```

Now if I try and pass question data in the `questions` field instead of `debuggingQuestions`:
```
Global variables:
{
"quiz": {
"id": 136,
"name": "fake name",
"description": "simple desc",
"creatorId": 1,
"media": "img.jpg",
"questions": [{
"media": "media",
"text": "text"
}]
}
}

Response:
{
"errors": [{
"message": "unhashable type: 'QuestionInput'",
"locations": [{
"line": 2,
"column": 3
}]
}],
"data": {
"createQuiz": null
}
}
```

What step am I missing so that `QuestionInput` is automatically converted into a Question sqlalchemy model?