SQLAlchemy 引发完整性错误，“表上的更新或删除违反了外键约束”

Question

我正在尝试在连接关系上设置级联删除。但是，每当我尝试删除应用程序中的帖子时，我都会收到一条错误消息，指出“表“帖子”上的更新或删除违反了外键约束””这是错误消息的照片：

这是我的代码：

class Post(db.Model):
    """Blog posts"""

    __tablename__ = "posts"

    id = db.Column(db.Integer,
                   primary_key=True,
                   autoincrement=True)
    title = db.Column(db.String(25),
                      nullable=False)
    content = db.Column(db.String(500),
                        nullable=False)
    created_at = db.Column(db.DateTime,
                           default=db.func.current_timestamp())
    user_table = db.Column(db.Integer, 
                           db.ForeignKey('users.id',
                                         ondelete='CASCADE'))

    tags = db.relationship('Tag', 
                            secondary="post_tags", 
                            back_populates="posts",
                            cascade="all, delete"
                            )


class Tag(db.Model):

    __tablename__ = "tags"

    id = db.Column(db.Integer,
                       primary_key=True,
                       autoincrement=True)
    name = db.Column(db.String,
                        unique=True)
    posts = db.relationship('Post',
                            secondary="post_tags",
                            back_populates="tags")
    
 

class Post_Tag(db.Model):

    __tablename__ = "post_tags"

    post_id = db.Column(db.Integer,
                        db.ForeignKey('posts.id'), primary_key=True)
    
    tag_id = db.Column(db.Integer,
                        db.ForeignKey('tags.id'), primary_key=True)

根据我查看过的文档和其他问题，我似乎设置正确。我在这里做错了什么？

更新 我可以删除标签，但无法删除帖子

Answer 1

You may be recieving this error because you're using backref instead of back_populates...

Also, I'd suggest defining your relationship bidirectionally, meaning in both the parent Post and child Tag models. This allows cascade deletion to the secondary table with different rules depending on which object is being deleted.

The following changes to your models should fix the error you're receiving:

# Modify your tags relationship to the following:
class Post(db.Model):
    ...
    tags = db.relationship(
               'Tag', 
               secondary="post_tags", 
               back_populates="posts", # use back-populates instead of backref
               cascade="all, delete"
           )


# Also, define your relationship from your tag model
class Tag(db.Model):
    __tablename__ = "tags"
    id = db.Column(db.Integer, primary_key=True, autoincrement=True)
    name = db.Column(db.String, unique=True)
    posts = db.relationship(
               'Post', 
               secondary="post_tags",
               back_populates="tags", # use back-populates instead of backref 
               # When a parent ("post") is deleted, don't delete the tags...
               passive_deletes=True
           )