SqlAlchemy：如何避免多对多关系中不方便的级联删除

如何解决SqlAlchemy：如何避免多对多关系中不方便的级联删除

我有两个表（多对多）映射到 sqlAlchemy 并配置了删除级联选项。

它工作正常，但我想改变一种行为。

实际行为

当我删除一个实体时，通过级联，该实体下的所有用户都将被删除。

+-------------+----------------------+
| id          | entities             |  
+-------------+----------------------+
|           1 | Entity_1             |   
|           2 | Entity_2             |   
+-------------+----------------------+

+-------------+----------------------+
| id          | users                |  
+-------------+----------------------+
|           1 | User_1               |   
+-------------+----------------------+

+-------------+----------------------+
| entity_id   | user_id              |  
+-------------+----------------------+
|           1 | 1                    |   
|           2 | 1                    |   
+-------------+----------------------+

当我删除实体 ID 2 时：

 session.query(EntityModel).filter(EntityModel.id == 2).delete()
 session.commit()

实际结果：

+-------------+----------------------+
| id          | entities             |  
+-------------+----------------------+
|           1 | Entity_1             |   
+-------------+----------------------+

+-------------+----------------------+
| id          | users                |  
+-------------+----------------------+
+-------------+----------------------+

+-------------+----------------------+
| entity_id   | user_id              |  
+-------------+----------------------+
+-------------+----------------------+

我不想要这种行为。

我需要从表实体中删除一个实体并删除您引用的用户，但是如果用户被两个（或多个）实体引用，我不想删除它，我只想删除引用表 entity_user 上，然后删除实体。

这在 sqlAlchemy 上可行吗？ EntityModel 类的关系配置如何？

class EntityModel(Base):

    __tablename__ = "entities"
    id = Column(Integer,primary_key=True,autoincrement=True)
    name = Column(String,nullable=False)


    users = relationship(
        "usermodel",secondary="entity_user",back_populates="entities",cascade="all,delete",)


class usermodel(Base):
    __tablename__ = "users"
    id = Column(Integer,autoincrement=True)
    name = Column(String)

    entities = relationship(
        "EntityModel",back_populates="users",uselist=True,lazy="joined",)


class Entityusermodel(Base):
    __tablename__ = "entity_user"
    __table_args__ = (PrimaryKeyConstraint("user_id","entity_id"),)
    user_id = Column(Integer,ForeignKey("users.id"))
    entity_id = Column(Integer,ForeignKey("entities.id"))

预期行为：

当我这样做时：

 session.query(EntityModel).filter(EntityModel.id == 2).delete()
 session.commit()

预期结果：

+-------------+----------------------+
| id          | entities             |  
+-------------+----------------------+
|           1 | Entity_1             |   
+-------------+----------------------+

+-------------+----------------------+
| id          | users                |  
+-------------+----------------------+
| 1           | User_1               |
+-------------+----------------------+

+-------------+----------------------+
| entity_id   | user_id              |  
+-------------+----------------------+
| 1           |1                     |
+-------------+----------------------+

Entity 1 和 User 1 的关系避免了 User 1 会被删除。