title: sqlalchemy.exc.InvalidRequestError: One or more mappers failed to initialize - can't proceed with initialization of other mappers
body: <p>This error happened when I tried to get access to the page. I didn't get errors when I created the tables, but seems like there are problems still.</p>
  
  <p>The models are like this:</p>
  
  <pre><code>class User(UserMixin, db.Model):
      id = db.Column(db.Integer, primary_key=True)
      username = db.Column(db.String(64), index=True, unique=True)
      sell_items = db.relationship('Item', backref='user')
  
  class Item(db.Model):
      id = db.Column(db.Integer, primary_key=True)
      item_name = db.Column(db.String(64), index=True)
      item_image = db.Column(db.String(200), index=True)
      price = db.Column(db.Float(10), index=True)
      user_id = db.Column(db.Integer, db.ForeignKey('user.id'))
      user = db.relationship('User', backref='sell_items')
  </code></pre>
  
  <p>The whole error message is this</p>
  
  <pre><code>Triggering mapper: 'Mapper|User|user'. Original exception was: Error creating backref 'user' on relationship 'User.sell_items': property of that name exists on mapper 'Mapper|Item|item'
  </code></pre>
  
  <p>How can I fix this? What I want to do is to refer to <code>username</code> who sells the item, but I cannot. There is a problem with the relationships between the models.</p>