SQLAlchemyでcreated_at, updated_atを共通化する

https://docs.sqlalchemy.org/en/13/orm/extensions/declarative/mixins.html

Mixinを使えば可能？

base.py

from sqlalchemy import Column, DateTime
from datetime import datetime


class TimestampMixin(object):
    created_at = Column(DateTime, default=datetime.now(), nullable=False)
    updated_at = Column(
        DateTime, default=datetime.now(), onupdate=datetime.now(), nullable=False
    )

articles.py

from sqlalchemy import Column, Integer, String

from .base import TimestampMixin

from ..db.engine import Base


class Article(Base, TimestampMixin):
    __tablename__ = "articles"

    id = Column(Integer, primary_key=True, index=True)
    title = Column(String)
    slug = Column(String, unique=True, index=True)
    description = Column(String)
    body = Column(String)

この方法だと実現できるが、DDLを見てみるとカラムが最初に作られてしまっていた。

CREATE TABLE articles (
	created_at TIMESTAMP WITHOUT TIME ZONE NOT NULL,
	updated_at TIMESTAMP WITHOUT TIME ZONE NOT NULL,
	id SERIAL NOT NULL,
	title VARCHAR,
	slug VARCHAR,
	description VARCHAR,
	body VARCHAR,
	PRIMARY KEY (id)
)

SQLAlchemyを簡単にまとめてみた（TIPSもあるよ） - Qiita

そんなときは@declared_attr を使うといいらしい

from sqlalchemy import Column, DateTime
from sqlalchemy.ext.declarative import declared_attr
from datetime import datetime


class TimestampMixin(object):
    @declared_attr
    def created_at(cls):
        return Column(DateTime, default=datetime.now(), nullable=False)

    @declared_attr
    def updated_at(cls):
        return Column(
            DateTime, default=datetime.now(), onupdate=datetime.now(), nullable=False
        )

生成したDDLを見て見ると意図したとおりに最後に来ている

CREATE TABLE articles (
	id SERIAL NOT NULL,
	title VARCHAR,
	slug VARCHAR,
	description VARCHAR,
	body VARCHAR,
	created_at TIMESTAMP WITHOUT TIME ZONE NOT NULL,
	updated_at TIMESTAMP WITHOUT TIME ZONE NOT NULL,
	PRIMARY KEY (id)
)