Hey there! Let's work through why your cascade deletes aren't kicking in with your association object setup. It's a common pitfall when switching from basic many-to-many association tables to association objects (which carry extra data like your attended field), so don't feel bad about missing this—SQLAlchemy's docs can be tricky to parse for edge cases like this.

The Core Issue #

Looking at your code, you're mixing up configuration for basic association tables (the ones that don't map to a model class) and association objects (your StudentClass model, which inherits from Base). Specifically:

1. You added a secondary parameter to the relationship definitions in Student and CMClass—this is only needed for plain association tables, not for association objects. This confuses SQLAlchemy's cascade logic.
2. While you set ondelete="CASCADE" on your foreign keys, you haven't told the ORM to respect or work with that database-level cascade properly.

Modified Working Code #

Here's how to adjust your models to get cascade deletes working:

Student Model

class Student(Base):
    __tablename__ = 'Students'
    id = Column('ID', Integer, primary_key=True, autoincrement=True)
    name = Column('NAME', String(50), unique=True)
    # Removed `secondary` parameter, added proper cascade and passive_deletes
    CMClasses = relationship(
        "StudentClass",
        back_populates="Student",
        cascade="all, delete-orphan",
        passive_deletes=True
    )

CMClass Model

class CMClass(Base):
    __tablename__ = "Classes"
    id = Column('ID', Integer, primary_key=True, autoincrement=True)
    name = Column('NAME', String(50), unique=True)
    classDate = Column('CLASS_DATE', DateTime, nullable=True)
    # Same fixes here: no secondary, cascade + passive_deletes
    Students = relationship(
        "StudentClass",
        back_populates="CMClass",
        cascade="all, delete-orphan",
        passive_deletes=True
    )

StudentClass Model (Unchanged)

Your association object model is already set up correctly with foreign keys and ondelete="CASCADE":

class StudentClass(Base):
    __tablename__ = 'StudentClasses'
    student_id = Column(Integer, ForeignKey('Students.ID', ondelete="CASCADE"), primary_key=True)
    CMClass_id = Column(Integer, ForeignKey('Classes.ID', ondelete="CASCADE"), primary_key=True)
    attended = Column(Boolean)
    Student = relationship("Student", back_populates="CMClasses")
    CMClass = relationship("CMClass", back_populates="Students")

What Changed & Why #

• Removed secondary: Since StudentClass is a full model class (not just a helper table), the relationship in Student/CMClass should directly point to StudentClass—no need to specify a secondary table. The back_populates handles the bidirectional link.
• cascade="all, delete-orphan": This tells SQLAlchemy's ORM to automatically delete StudentClass instances when their parent Student or CMClass is deleted. The delete-orphan part also handles cases where you remove a StudentClass from the parent's collection (e.g., student.CMClasses.remove(some_student_class)).
• passive_deletes=True: This works with your ondelete="CASCADE" on the foreign keys. It tells SQLAlchemy to let the database handle the cascade delete instead of trying to generate extra DELETE statements for the association objects, which avoids conflicts and is more efficient.

Testing the Fix #

To verify it works, try this simple test:

# Fetch a student and delete them
student = session.get(Student, 1)
session.delete(student)
session.commit()

# Check the StudentClasses table—all entries linked to this student should be gone!

This setup will ensure that deleting a Student or CMClass automatically cleans up all related StudentClass records, both via ORM-level cascades and database-level safety nets.

内容的提问来源于stack exchange，提问作者TheFaithfulLearner