Flask-RESTful API Returns 500 scenarios on Nested JSON Validation with Marshmallow 3.0
I'm working on a Flask-RESTful API that uses Marshmallow for data validation. Recently, I encountered an scenario where the API returns a 500 Internal Server behavior when I try to post nested JSON data. The expected structure of the request payload is as follows: ```json { "user": { "username": "johndoe", "email": "john@example.com", "profile": { "age": 30, "bio": "Software Developer" } } } ``` I have defined my schemas like this: ```python from marshmallow import Schema, fields, post_load class ProfileSchema(Schema): age = fields.Int(required=True) bio = fields.Str(required=True) class UserSchema(Schema): username = fields.Str(required=True) email = fields.Email(required=True) profile = fields.Nested(ProfileSchema) @post_load def make_user(self, data, **kwargs): return User(**data) ``` The `User` model is defined properly, and I have handled the creation in my resource method like this: ```python from flask_restful import Resource class UserResource(Resource): def post(self): json_data = request.get_json() errors = UserSchema().validate(json_data) if errors: return {'errors': errors}, 422 user = UserSchema().load(json_data) db.session.add(user) db.session.commit() return {'message': 'User created successfully'}, 201 ``` When I attempt to send a POST request with the nested JSON structure, I receive a 500 behavior without much detail in the logs. The only traceback I see is: ``` AttributeError: 'NoneType' object has no attribute 'username' ``` I've verified that the JSON is being received correctly by printing `json_data`. The scenario seems to occur during the loading of the data with `UserSchema().load(json_data)`, but I need to pinpoint the exact cause. I've tried printing the `errors` dictionary, but it returns empty when I validate the input. Could someone guide to understand what might be going wrong with the nested validation? I'm using Flask 2.0 and Marshmallow 3.0. I've been using Python for about a year now. Hoping someone can shed some light on this.