I just came across a solution to this problem. Adding 'with_polymorphic': 
'*' to __mapper_args__ on the Node model was enough.

class Node(db.Model):
   id = db.Column(db.Integer, primary_key=True)
   type = db.Column(db.String(20))
   name = db.Column(db.String(30), nullable=False)
   date_added = db.Column(db.DateTime, default=datetime.now())
   parent_id = db.Column(db.Integer, db.ForeignKey('node.id'), nullable=True
)
   path = db.Column(db.String(200), default='')
   __mapper_args__ = {'polymorphic_on': type,
                      'with_polymorphic': '*' }


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