title: sqlalchemy.orm.exc.UnmappedInstanceError: Class 'builtins.NoneType' is not mapped
body: <p>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 <code>sqlalchemy.orm.exc.UnmappedInstanceError: Class 'builtins.NoneType' is not mapped</code>.</p>
  
  <p>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 <code>user_x.task[3].subtask[1]</code></p>
  
  <p>Here is the code,
  (models, script, error)</p>
  
  <p>models</p>
  
  <pre><code>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)
  </code></pre>
  
  <p>test_script.py</p>
  
  <pre><code>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()
  </code></pre>
  
  <p>error message</p>
  
  <pre><code>/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 &lt;module&gt;
      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
  </code></pre>
  
  <p>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.</p>