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"""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.
"""
from sqlalchemy import Column
from sqlalchemy import create_engine
from sqlalchemy import event
from sqlalchemy import ForeignKey
from sqlalchemy import Integer
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 make_transient
from sqlalchemy.orm import relationship
from sqlalchemy.orm import Session
from sqlalchemy.orm import sessionmaker
class Versioned:
def new_version(self, session):
# make us transient (removes persistent
# identity).
make_transient(self)
# set 'id' to None.
# a new PK will be generated on INSERT.
self.id = None
@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"
id = Column(Integer, primary_key=True)
data = Column(String)
Base.metadata.create_all(engine)
session = Session()
e1 = Example(data="e1")
session.add(e1)
session.commit()
e1.data = "e2"
session.commit()
assert session.query(Example.id, Example.data).order_by(Example.id).all() == (
[(1, "e1"), (2, "e2")]
)
# example 2, versioning with a parent
class Parent(Base):
__tablename__ = "parent"
id = Column(Integer, primary_key=True)
child_id = Column(Integer, ForeignKey("child.id"))
child = relationship("Child", backref=backref("parent", uselist=False))
class Child(Versioned, Base):
__tablename__ = "child"
id = Column(Integer, primary_key=True)
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
self.parent.child = self
Base.metadata.create_all(engine)
session = Session()
p1 = Parent(child=Child(data="c1"))
session.add(p1)
session.commit()
p1.child.data = "c2"
session.commit()
assert p1.child_id == 2
assert session.query(Child.id, Child.data).order_by(Child.id).all() == (
[(1, "c1"), (2, "c2")]
)
|
