sqlalchemy.orm.exc.UnmappedInstanceError: Class 'builtins.dict' is not mapped

I'm trying to insert a new User into a DB using SQLAlchemy and Marshmallow. The user parameter is received from an API endpoint. Everything works until I get to this line in the create function:

db.session.add(new_user)

The value of the new_user variable at that point is:

{'password': 'string', 'email': 'fave@string', 'username': 'string'}

Function:

def create(user):
    uname = user.get('username')
    email = user.get('email')
    password = user.get ('password')

    existing_username = User.query.filter(User.username == uname).one_or_none()

    if existing_username is None:

        schema = UserSchema()
        new_user = schema.load(user, session=db.session)

        db.session.add(new_user) <- It fails here
        db.session.commit()

        return schema.dump(new_user), 201

Models:

    class User (db.Model):
        id = db.Column(db.Integer, primary_key=True)
        username = db.Column(db.String(20), unique=True, nullable=False)
        email = db.Column(db.String(120), unique=True, nullable=False)
        profile_picture = db.Column(db.String(20), nullable=False, default='default.jpg')
        password = db.Column(db.String(60), nullable=False)
        creation_date = db.Column(db.DateTime(120), nullable=False, default=datetime.utcnow)
        updated_date = db.Column(db.DateTime(120), nullable=False, default=datetime.utcnow, onupdate=datetime.utcnow)
        posts = db.relationship('Post', backref='author', lazy=True)

        def __repr__(self):
            return f"User ('{self.username}','{self.email}','{self.profile_picture}') "

    class Post (db.Model):
        id = db.Column(db.Integer, primary_key=True)
        title = db.Column(db.String(100), nullable=False)
        date_posted = db.Column(db.DateTime(120), nullable=False, default=datetime.utcnow)
        content = db.Column(db.Text, nullable=False)
        user_id = db.Column(db.Integer, db.ForeignKey('user.id'), nullable=False)

        def __repr__(self):
            return f"Post ('{self.title}','{self.date_posted}') "

Schema:

class UserSchema(ma.SQLAlchemyAutoSchema):
    class Meta:
        model = User
        sqla_session = db.session

Part of the console errors that I think are relevant:

127.0.0.1 - - [14/May/2020 21:33:35] "POST /api/v1/users HTTP/1.1" 500 -
Traceback (most recent call last):
  File "/Users/user/.local/share/virtualenvs/apis-JEynsq5i-/Users/user/.pyenv/shims/python/lib/python3.6/site-packages/sqlalchemy/orm/session.py", line 1975, in add
    state = attributes.instance_state(instance)
AttributeError: 'dict' object has no attribute '_sa_instance_state'

The above exception was the direct cause of the following exception:
.
.
.
File "/Users/user/.local/share/virtualenvs/apis-JEynsq5i-/Users/user/.pyenv/shims/python/lib/python3.6/site-packages/connexion/decorators/parameter.py", line 121, in wrapper
    return function(**kwargs)
  File "/Users/user/development/flask/apis/src/users.py", line 100, in create
    db.session.add(new_user)
  File "/Users/user/.local/share/virtualenvs/apis-JEynsq5i-/Users/user/.pyenv/shims/python/lib/python3.6/site-packages/sqlalchemy/orm/scoping.py", line 162, in do
    return getattr(self.registry(), name)(*args, **kwargs)
  File "/Users/user/.local/share/virtualenvs/apis-JEynsq5i-/Users/user/.pyenv/shims/python/lib/python3.6/site-packages/sqlalchemy/orm/session.py", line 1978, in add
    exc.UnmappedInstanceError(instance), replace_context=err,
  File "/Users/user/.local/share/virtualenvs/apis-JEynsq5i-/Users/user/.pyenv/shims/python/lib/python3.6/site-packages/sqlalchemy/util/compat.py", line 178, in raise_
    raise exception
sqlalchemy.orm.exc.UnmappedInstanceError: Class 'builtins.dict' is not mapped

I'm not sure why is throwing that Class 'builtins.dict' is not mapped error. Any tips?

@IljaEverilä thanks, that helped me to resolve the issue.

I've also found a similar issue at: https://github.com/marshmallow-code/marshmallow/issues/630

The suggestion was to use marshmallow-sqlalchemy.

I was using the Marshmallow object created from a config file as:

config.py:

from flask_marshmallow import Marshmallow
ma = Marshmallow(app)

My UserSchema was:

from .config import db, ma
.
.
.
class UserSchema(ma.SQLAlchemyAutoSchema):
    class Meta:
        model = User
        sqla_session = db.session

And updated it to:

from marshmallow_sqlalchemy import SQLAlchemyAutoSchema, auto_field

class UserSchema(SQLAlchemyAutoSchema):
    class Meta:
        model = User
        include_relationships = True
        load_instance = True

As seen in:

https://github.com/marshmallow-code/marshmallow-sqlalchemy#generate-marshmallow-schemas

It's working now.