新增databases.py把所有建表基类统一成一个，才可以链接外键。

class_management/model/class_management_model.py

@@ -1,9 +1,10 @@
+from databases import engine_all,Base_all,Session
 from sqlalchemy import Column, Integer, String, Date, ForeignKey
 from sqlalchemy.dialects.mysql import DATETIME
 from datetime import datetime
 from class_management.database import Base
 
-class ClassInfo(Base):
+class ClassInfo(Base_all):
     __tablename__ = 'classes'
 
     id = Column(Integer