如何在模型中使用"删除限制"？

Question

我已经想出如何使用"删除级联",但不清楚如何做"删除限制"约束.我想要实现的是无法删除具有子记录或子记录的父级.

class User(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    name = db.Column(db.String)
    fullname = db.Column(db.String)
    password = db.Column(db.String)

    posts = db.relationship("Post", backref='user', cascade="all, delete, delete-orphan")

class Post(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    title = db.Column(db.String, nullable=False)
    description = db.Column(db.String, nullable=False)
    user_id = db.Column(db.Integer, db.ForeignKey('users.id', onupdate="CASCADE", ondelete="CASCADE"))

CREATE TABLE posts (
    id INTEGER NOT NULL,
    title VARCHAR NOT NULL,
    description VARCHAR NOT NULL,
    user_id INTEGER,
    PRIMARY KEY (id),
    FOREIGN KEY(user_id) REFERENCES users (id) ON DELETE RESTRICT ON UPDATE CASCADE
);

CREATE TABLE users (
    id INTEGER NOT NULL,
    name VARCHAR,
    fullname VARCHAR,
    password VARCHAR,
    PRIMARY KEY (id)
);

用"restrict"替换"delete"只允许我删除父项并保留孤立的行.

如何正确指定"限制"行为？

Answer 1

默认情况下,SQLite不支持外键约束. 必须在编译时启用它们并在运行时启用它们,否则它们将被静默忽略.

您可以通过pragma foreign_keys在sqlite shell中运行来检查是否启用了外键.如果返回1,则启用它们.如果返回0,则禁用它们.如果它没有返回任何内容,则不支持它们,并且必须重新编译sqlite以支持它们.

如果禁用了外键,则可以指示SQLAlchemy在创建连接时启用它们.

from sqlalchemy import event
from sqlalchemy.engine import Engine
from sqlite3 import Connection as SQLite3Connection

@event.listens_for(Engine, "connect")
def _set_sqlite_pragma(dbapi_connection, connection_record):
    if isinstance(dbapi_connection, SQLite3Connection):
        cursor = dbapi_connection.cursor()
        cursor.execute("PRAGMA foreign_keys=ON;")
        cursor.close()

_{来源:https://stackoverflow.com/a/15542046/400617}

SQL 外键级联不同于SQLAlchemy的关系级联(向下滚动第二个链接以查看两者的详细比较).SQLAlchemy没有"限制"级联.您应该在外键上指定它.如果更改已存在的外键,请务必重新创建/迁移数据库.

class Post(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    user_id = db.Column(db.Integer, db.ForeignKey(User.id, ondelete='RESTRICT'), nullable=False)