class Book(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    name = db.Column(db.String(16))
    pages = db.relationship('Page')

class Page(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    book_id = db.Column(db.Integer, db.ForeignKey('book.id'))
    num = db.Column(db.Integer)
    symbols = db.relationship('Symbol')

class Symbol(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    page_id = db.Column(db.Integer, db.ForeignKey('page.id'))
    symbol = db.Column(db.String(1))