SQLAlchemyでbefore_deleteイベントを使用して関連オブジェクトを自動削除する

SQLAlchemy で条件に基づいてオブジェクトを自動削除する方法

cascade="delete" オプションを使用すると、親オブジェクトが削除されると、関連する子オブジェクトも自動的に削除されます。これは、一対多関係や多対多関係で役立ちます。

from sqlalchemy import create_engine
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy import Column, Integer, String, ForeignKey

engine = create_engine("sqlite:///database.db")
Base = declarative_base()

class User(Base):
    __tablename__ = "users"
    id = Column(Integer, primary_key=True)
    name = Column(String(255))

class Post(Base):
    __tablename__ = "posts"
    id = Column(Integer, primary_key=True)
    title = Column(String(255))
    user_id = Column(Integer, ForeignKey("users.id"))

Base.metadata.create_all(engine)

user = User(name="John Doe")
post1 = Post(title="My first post", user=user)
post2 = Post(title="My second post", user=user)

engine.session.add(user)
engine.session.add(post1)
engine.session.add(post2)
engine.session.commit()

# Delete the user
engine.session.delete(user)
engine.session.commit()

# Check if the posts were also deleted
posts = engine.session.query(Post).all()
assert len(posts) == 0

before_delete イベントを使用すると、オブジェクトが削除される前にカスタムロジックを実行できます。このイベントを使用して、関連するオブジェクトを削除したり、その他の処理を実行したりできます。

from sqlalchemy import create_engine
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy import Column, Integer, String, ForeignKey
from sqlalchemy.orm import event

engine = create_engine("sqlite:///database.db")
Base = declarative_base()

class User(Base):
    __tablename__ = "users"
    id = Column(Integer, primary_key=True)
    name = Column(String(255))

class Post(Base):
    __tablename__ = "posts"
    id = Column(Integer, primary_key=True)
    title = Column(String(255))
    user_id = Column(Integer, ForeignKey("users.id"))

@event.before_delete(User)
def before_user_delete(mapper, connection, target):
    # Delete all related posts
    posts = engine.session.query(Post).filter_by(user_id=target.id).all()
    for post in posts:
        engine.session.delete(post)

Base.metadata.create_all(engine)

user = User(name="John Doe")
post1 = Post(title="My first post", user=user)
post2 = Post(title="My second post", user=user)

engine.session.add(user)
engine.session.add(post1)
engine.session.add(post2)
engine.session.commit()

# Delete the user
engine.session.delete(user)
engine.session.commit()

# Check if the posts were also deleted
posts = engine.session.query(Post).all()
assert len(posts) == 0

from sqlalchemy import create_engine
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy import Column, Integer, String, ForeignKey

engine = create_engine("sqlite:///database.db")
Base = declarative_base()

class User(Base):
    __tablename__ = "users"
    id = Column(Integer, primary_key=True)
    name = Column(String(255))

class Post(Base):
    __tablename__ = "posts"
    id = Column(Integer, primary_key=True)
    title = Column(String(255))
    user_id = Column(Integer, ForeignKey("users.id"), ondelete="CASCADE")

Base.metadata.

cascade="delete"

from sqlalchemy import create_engine
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy import Column, Integer, String, ForeignKey

engine = create_engine("sqlite:///database.db")
Base = declarative_base()

class User(Base):
    __tablename__ = "users"
    id = Column(Integer, primary_key=True)
    name = Column(String(255))

class Post(Base):
    __tablename__ = "posts"
    id = Column(Integer, primary_key=True)
    title = Column(String(255))
    user_id = Column(Integer, ForeignKey("users.id", ondelete="CASCADE"))

Base.metadata.create_all(engine)

user = User(name="John Doe")
post1 = Post(title="My first post", user=user)
post2 = Post(title="My second post", user=user)

engine.session.add(user)
engine.session.add(post1)
engine.session.add(post2)
engine.session.commit()

# Delete the user
engine.session.delete(user)
engine.session.commit()

# Check if the posts were also deleted
posts = engine.session.query(Post).all()
assert len(posts) == 0

説明:

• このコードは、sqlite:///database.db という名前の SQLite データベースに接続します。
• User と Post という 2 つのテーブルを作成します。
• User テーブルには、id と name という 2 つの列があります。
• Post テーブルには、id、title、user_id という 3 つの列があります。
• user_id 列は User テーブルの id 列を参照する外部キーです。
• コードは、John Doe という名前の User オブジェクトと、そのユーザーに関連する 2 つの Post オブジェクトを作成します。
• コードは、User オブジェクトを削除します。
• コードは、Post テーブルが空であることを確認します。

before_delete event

from sqlalchemy import create_engine
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy import Column, Integer, String, ForeignKey
from sqlalchemy.orm import event

engine = create_engine("sqlite:///database.db")
Base = declarative_base()

class User(Base):
    __tablename__ = "users"
    id = Column(Integer, primary_key=True)
    name = Column(String(255))

class Post(Base):
    __tablename__ = "posts"
    id = Column(Integer, primary_key=True)
    title = Column(String(255))
    user_id = Column(Integer, ForeignKey("users.id"))

@event.before_delete(User)
def before_user_delete(mapper, connection, target):
    # Delete all related posts
    posts = engine.session.query(Post).filter_by(user_id=target.id).all()
    for post in posts:
        engine.session.delete(post)

Base.metadata.create_all(engine)

user = User(name="John Doe")
post1 = Post(title="My first post", user=user)
post2 = Post(title="My second post", user=user)

engine.session.add(user)
engine.session.add(post1)
engine.session.add(post2)
engine.session.commit()

# Delete the user
engine.session.delete(user)
engine.session.commit()

# Check if the posts were also deleted
posts = engine.session.query(Post).all()
assert len(posts) == 0

• Post テーブルには、id、title、`user_

SQLAlchemy でオブジェクトを自動削除するその他の方法

declarative_base.registry.delete_cascade を使用すると、グローバル設定で cascade="delete" オプションを有効にすることができます。これにより、すべての関連オブジェクトが親オブジェクトが削除されると自動的に削除されます。

from sqlalchemy import create_engine
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy import Column, Integer, String, ForeignKey

engine = create_engine("sqlite:///database.db")
Base = declarative_base()

declarative_base.registry.delete_cascade = True

class User(Base):
    __tablename__ = "users"
    id = Column(Integer, primary_key=True)
    name = Column(String(255))

class Post(Base):
    __tablename__ = "posts"
    id = Column(Integer, primary_key=True)
    title = Column(String(255))
    user_id = Column(Integer, ForeignKey("users.id"))

Base.metadata.create_all(engine)

user = User(name="John Doe")
post1 = Post(title="My first post", user=user)
post2 = Post(title="My second post", user=user)

engine.session.add(user)
engine.session.add(post1)
engine.session.add(post2)
engine.session.commit()

# Delete the user
engine.session.delete(user)
engine.session.commit()

# Check if the posts were also deleted
posts = engine.session.query(Post).all()
assert len(posts) == 0

relationship() オプションの cascade パラメータを使用して、関連オブジェクトが削除されるかどうかを指定できます。

from sqlalchemy import create_engine
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy import Column, Integer, String, ForeignKey
from sqlalchemy.orm import relationship

engine = create_engine("sqlite:///database.db")
Base = declarative_base()

class User(Base):
    __tablename__ = "users"
    id = Column(Integer, primary_key=True)
    name = Column(String(255))

class Post(Base):
    __tablename__ = "posts"
    id = Column(Integer, primary_key=True)
    title = Column(String(255))
    user_id = Column(Integer, ForeignKey("users.id"))

user = relationship("posts", cascade="all, delete-orphan")

Base.metadata.create_all(engine)

user = User(name="John Doe")
post1 = Post(title="My first post", user=user)
post2 = Post(title="My second post", user=user)

engine.session.add(user)
engine.session.add(post1)
engine.session.add(post2)
engine.session.commit()

# Delete the user
engine.session.delete(user)
engine.session.commit()

# Check if the posts were also deleted
posts = engine.session.query(Post).all()
assert len(posts) == 0

mappers.delete 関数を使用して、オブジェクトを明示的に削除できます。この関数は、関連オブジェクトも削除するように設定できます。

from sqlalchemy import create_engine
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy import Column, Integer, String, ForeignKey
from sqlalchemy.orm import mappers

engine = create_engine("sqlite:///database.db")
Base = declarative_base()

class User(Base):
    __tablename__ = "users"
    id = Column(Integer, primary_key=True)
    name = Column(String(255))

class Post(Base):
    __tablename__ = "posts"
    id = Column(Integer, primary_key=True)
    title = Column(String(255))
    user_id = Column(Integer, ForeignKey("users.id"))

def delete_user(user):
    # Delete all related posts
    for post in user.posts:
        mappers.delete(post)

    # Delete the user
    mappers.delete(user)

Base.metadata.create_all(engine)

user = User(name="