sqlalchemy.exc.CompileError: (in table 'user', column 'username'): VARCHAR requires a length on dialect mysql

映射数据库时报错：sqlalchemy.exc.CompileError: (in table 'user', column 'username'): VARCHAR requires a length on dialect mysql

错误代码：

username = db.Column(db.String)

String需要指定长度，如：

username = db.Column(db.String(50))