新增databases.py把所有建表基类统一成一个，才可以链接外键。

databases.py

@@ -0,0 +1,35 @@
+# databases链接数据库
+
+from sqlalchemy import *
+from sqlalchemy.orm import sessionmaker,declarative_base
+
+# 导入.env参数
+import os
+from dotenv import load_dotenv
+load_dotenv( )
+DB_USER = os.getenv("DB_USER", "root")
+DB_PASSWORD = os.getenv("DB_PASSWORD", "")
+DB_HOST = os.getenv("DB_HOST", "localhost")
+DB_PORT = os.getenv("DB_PORT", "3306")
+DB_NAME = os.getenv("DB_NAME", "student_manage_system")
+
+db_url = f"mysql+pymysql://{DB_USER}:{DB_PASSWORD}@{DB_HOST}:{DB_PORT}/{DB_NAME}?charset=utf8mb4"
+
+engine_all = create_engine(db_url)
+
+Base_all = declarative_base()
+
+Session = sessionmaker(bind=engine_all
+                               ,autoflush=False
+                               ,autocommit=False)
+
+def get_db():
+    db = Session()
+    try:
+        yield db
+    finally:
+        db.close()
+
+
+
+