当前上下文为空时如何处理sqlalchemy onupdate？

Question

我有一个文章模型，根据它的标题会有 slug，模型是这样的：

from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy import Column, Integer, String, Text

Base = declarative_base()


class Article(Base):

    __tablename__ = 'article'

    id = Column(Integer, primary_key=True)
    title = Column(String(100), nullable=False)
    content = Column(Text)
    slug = Column(String(100), nullable=False,
                  default=lambda c: c.current_params['title'],
                  onupdate=lambda c: c.current_params['title'])

slug 正在获取标题的值。所以，每次文章slug 都会匹配它的标题。但是，当我在不更改标题的情况下编辑内容时，这 引发异常

(builtins.KeyError) 'title' [SQL: 'UPDATE article SET content=?, slug=?,
updated_at=? WHERE article = ?'] [parameters: [{'article_id': 1,
'content': 'blah blah blah'}]]

我猜是因为current_params 不包含title。如果，我改变 那里的lambda 并使用if，slug 将是None。那我该如何处理 这并保持 slug 值匹配它的标题？

Answer 1

你可以使用validates() decorator:

from sqlalchemy.orm import validates

class Article(db.Model):
    __tablename__ = 'article'
    id = db.Column(db.Integer, primary_key=True)
    title = db.Column(db.String(100), nullable=False)
    content = db.Column(db.String)
    slug = db.Column(db.String(100), nullable=False)

    @validates('title')
    def update_slug(self, key, title):
        self.slug = title
        return title

或events:

from sqlalchemy import event

class Article(db.Model):
    __tablename__ = 'article'
    id = db.Column(db.Integer, primary_key=True)
    title = db.Column(db.String(100), nullable=False)
    content = db.Column(db.String)
    slug = db.Column(db.String(100), nullable=False)

@event.listens_for(Article.title, 'set')
def update_slug(target, value, oldvalue, initiator):
    target.slug = value