examples.versioned_rows.versioned_rows_w_versionid — SQLAlchemy 2.0 Documentation

Release: 2.0.48 | Release Date: March 2, 2026

# SQLAlchemy 2.0 Documentation

# Source code for examples.versioned_rows.versioned_rows_w_versionid

    """Illustrates a method to intercept changes on objects, turning
    an UPDATE statement on a single row into an INSERT statement, so that a new
    row is inserted with the new data, keeping the old row intact.

    This example adds a numerical version_id to the Versioned class as well
    as the ability to see which row is the most "current" version.

    """

    from sqlalchemy import Boolean
    from sqlalchemy import Column
    from sqlalchemy import create_engine
    from sqlalchemy import event
    from sqlalchemy import ForeignKeyConstraint
    from sqlalchemy import func
    from sqlalchemy import Integer
    from sqlalchemy import select
    from sqlalchemy import String
    from sqlalchemy.ext.declarative import declarative_base
    from sqlalchemy.orm import attributes
    from sqlalchemy.orm import backref
    from sqlalchemy.orm import column_property
    from sqlalchemy.orm import make_transient
    from sqlalchemy.orm import relationship
    from sqlalchemy.orm import Session
    from sqlalchemy.orm import sessionmaker

    class Versioned:
        # we have a composite primary key consisting of "id"
        # and "version_id"
        id = Column(Integer, primary_key=True)
        version_id = Column(Integer, primary_key=True, default=1)

        # optional - add a persisted is_current_version column
        is_current_version = Column(Boolean, default=True)

        # optional - add a calculated is_current_version column
        @classmethod
        def __declare_last__(cls):
            alias = cls.__table__.alias()
            cls.calc_is_current_version = column_property(
                select(func.max(alias.c.version_id) == cls.version_id).where(
                    alias.c.id == cls.id
                )
            )

        def new_version(self, session):
            # optional - set previous version to have is_current_version=False
            old_id = self.id
            session.query(self.__class__).filter_by(id=old_id).update(
                values=dict(is_current_version=False), synchronize_session=False
            )

            # make us transient (removes persistent
            # identity).
            make_transient(self)

            # increment version_id, which means we have a new PK.
            self.version_id += 1

    @event.listens_for(Session, "before_flush")
    def before_flush(session, flush_context, instances):
        for instance in session.dirty:
            if not isinstance(instance, Versioned):
                continue
            if not session.is_modified(instance):
                continue

            if not attributes.instance_state(instance).has_identity:
                continue

            # make it transient
            instance.new_version(session)

            # re-add
            session.add(instance)

    Base = declarative_base()

    engine = create_engine("sqlite://", echo=True)

    Session = sessionmaker(engine)

    # example 1, simple versioning

    class Example(Versioned, Base):
        __tablename__ = "example"
        data = Column(String)

    Base.metadata.create_all(engine)

    session = Session()
    e1 = Example(id=1, data="e1")
    session.add(e1)
    session.commit()

    e1.data = "e2"
    session.commit()

    assert session.query(
        Example.id,
        Example.version_id,
        Example.is_current_version,
        Example.calc_is_current_version,
        Example.data,
    ).order_by(Example.id, Example.version_id).all() == (
        [(1, 1, False, False, "e1"), (1, 2, True, True, "e2")]
    )

    # example 2, versioning with a parent

    class Parent(Base):
        __tablename__ = "parent"
        id = Column(Integer, primary_key=True)
        child_id = Column(Integer)
        child_version_id = Column(Integer)
        child = relationship("Child", backref=backref("parent", uselist=False))

        __table_args__ = (
            ForeignKeyConstraint(
                ["child_id", "child_version_id"], ["child.id", "child.version_id"]
            ),
        )

    class Child(Versioned, Base):
        __tablename__ = "child"

        data = Column(String)

        def new_version(self, session):
            # expire parent's reference to us
            session.expire(self.parent, ["child"])

            # create new version
            Versioned.new_version(self, session)

            # re-add ourselves to the parent.  this causes the
            # parent foreign key to be updated also
            self.parent.child = self

    Base.metadata.create_all(engine)

    session = Session()

    p1 = Parent(child=Child(id=1, data="c1"))
    session.add(p1)
    session.commit()

    p1.child.data = "c2"
    session.commit()

    assert p1.child_id == 1
    assert p1.child.version_id == 2

    assert session.query(
        Child.id,
        Child.version_id,
        Child.is_current_version,
        Child.calc_is_current_version,
        Child.data,
    ).order_by(Child.id, Child.version_id).all() == (
        [(1, 1, False, False, "c1"), (1, 2, True, True, "c2")]
    )