title: Getting sqlalchemy.exc.InvalidRequestError when trying to setup an one to many relationship
body: <p>I have 2 tables that I`m trying to setup with this relationship but when I try to run the app i get:</p>
  <blockquote>
  <p>sqlalchemy.exc.InvalidRequestError: One or more mappers failed to
  initialize - can't proceed with initialization of other mappers.</p>
  </blockquote>
  <pre><code>class Items(Base):
      __tablename__ = 'Item'
  
      ID_ITEM = Column(Integer, primary_key=True)
      NAME = Column(String(80, 'SQL_Latin1_General_CP1_CI_AS'), nullable=False)
  
      addresses = relationship('Unit', backref='items', lazy=True)
  
  </code></pre>
  <pre><code>class Unit(Base):
      __tablename__ = 'Units'
  
      ID_UNIT = Column(Integer, primary_key=True)
      VALUE = Column(Numeric(12, 2), nullable=False)
  
      item_id = Column(Integer, ForeignKey('item.id'), nullable=False)
  
  </code></pre>
  <p>What am I doing wrong here?</p>