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

Question

I have 3 sqlalchemy models setup which are all one to many relationships. The User model contains many Task models and the Task model contains many Subtask models. When I execute the test_script.py I get the error sqlalchemy.orm.exc.UnmappedInstanceError: Class 'builtins.NoneType' is not mapped.

I have read and tried a few different relationships on each module. My goal is to be able to have lists in the User and Task models containing their correct children. I would eventually like to access list items such as user_x.task[3].subtask[1]

Here is the code, (models, script, error)

models

class User(Base):
    """ User Model for storing user related details """
    __tablename__ = 'Users'

    id = Column(Integer, primary_key=True, autoincrement=True, nullable=False)
    username = Column(String(255), nullable=False)
    password_hash = Column(String(128), nullable=False)
    email = Column(String(255), nullable=False, unique=True)
    created_date = Column(DateTime, default=datetime.utcnow)

    tasks = relationship(Task, backref=backref("Users", uselist=False))

    def __init__(self, username: str, password_hash: str, email: str):
        self.username = username
        self.password_hash = password_hash
        self.email = email



class Task(Base):
    """ Task Model for storing task related details """
    __tablename__ = 'Tasks'

    id = Column(Integer, primary_key=True, autoincrement=True, nullable=False)
    title = Column(String(255), nullable=False)
    folder_name = Column(String(255), nullable=True)
    due_date = Column(DateTime, nullable=True)
    starred = Column(Boolean, default=False)
    completed = Column(Boolean, default=False)
    note = Column(String, nullable=True)
    created_date = Column(DateTime, default=datetime.utcnow)
    user_id = Column(Integer, ForeignKey('Users.id'))

    subtasks = relationship(Subtask, backref=backref("Tasks", uselist=False))

    def __init__(self, title: str, **kwargs):
        self.title = title
        self.folder_name = kwargs.get('folder_name', None)
        self.due_date = kwargs.get('due_date', None)
        self.starred = kwargs.get('starred', False)
        self.completed = kwargs.get('completed', False)
        self.note = kwargs.get('note', None)



class Subtask(Base):
    """ Subtask Model for storing subtask related details """
    __tablename__ = 'Subtasks'

    id = Column(Integer, primary_key=True, nullable=False, autoincrement=True)
    title = Column(String(255), nullable=False)
    completed = Column(Boolean, default=False)
    task_id = Column(Integer, ForeignKey('Tasks.id'))

    def __init__(self, title: str, **kwargs):
        self.title = title
        self.completed = kwargs.get('completed', False)

test_script.py

session1 = create_session()

user1 = User(
    username="Stephen",
    password_hash="p-hash",
    email="stephen@gmail.com"
).tasks.append(
    Task(
        title='Delete Me',
        folder_name='Folder 1'
    ).subtasks.append(
        Subtask(
            title='Delete Me 2'
        )
    )
)

session1.add(user1)

session1.commit()
session1.close()

error message

/Users/StephenCollins/Respositories/pycharm_workspace/Personal/BusyAPI_v1/venv/bin/python /Users/StephenCollins/Respositories/pycharm_workspace/Personal/BusyAPI_v1/tests/test_script.py
Traceback (most recent call last):
  File "/Users/StephenCollins/Respositories/pycharm_workspace/Personal/BusyAPI_v1/venv/lib/python3.7/site-packages/sqlalchemy/orm/session.py", line 1943, in add
    state = attributes.instance_state(instance)
AttributeError: 'NoneType' object has no attribute '_sa_instance_state'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/Users/StephenCollins/Respositories/pycharm_workspace/Personal/BusyAPI_v1/tests/test_script.py", line 24, in <module>
    session1.add(user1)
  File "/Users/StephenCollins/Respositories/pycharm_workspace/Personal/BusyAPI_v1/venv/lib/python3.7/site-packages/sqlalchemy/orm/session.py", line 1945, in add
    raise exc.UnmappedInstanceError(instance)
sqlalchemy.orm.exc.UnmappedInstanceError: Class 'builtins.NoneType' is not mapped

Hopefully I am close but if I am completely in the wrong direction just let me know. I used Hibernate a little bit in my prior internship but I am self taught (the best kind of taught) in python, sqlalchemy, and mysql.

Answer 1

This typically happens in delete/add operations in sqlalchemy where all items are deleted at the same time. The way to solve it is to add/delete with a loop:

To delete multiple elements do

items_2_delete = postgres_db.query(FilterType).all()
    for item in items_2_delete:
        session_db.delete(item)
session_db.commit()

, instead of

postgres_db.query(FilterType).delete()

Similarly, to add multiple elements do:

items = ['item1', 'item2']
for item in items:
    session_db.add(item)
    session_db.commit()

instead of

session_db.add(items)
session_db.commit()

Answer 2

In Python methods that mutate an object in-place usually return None. Your assignment's expression chains creating the model object, accessing a relationship attribute, and appending to said attribute, which returns None. Split the operations:

user1 = User(
    username="Stephen",
    password_hash="p-hash",
    email="stephen@gmail.com"
)

task1 = Task(
    title='Delete Me',
    folder_name='Folder 1'
)

task1.subtasks.append(
    Subtask(
        title='Delete Me 2'
    )
)

user1.tasks.append(task1)

Answer 3

since you're overloading __init__ it's necessary to initialize the superclass in your models:

class User(Base):
    def __init__(self, username: str, password_hash: str, email: str):
        // Initialize self
        [...]

        // Initialize base class
        super(User, self).__init__()

Note that it's not generally necessary to define your own constructor __init__ method because the declarative base class already defines a constructor that takes a **kwargs corresponding to the model properties.