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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.orm import *
from sqlalchemy import *
from sqlalchemy.orm.interfaces import SessionExtension
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import attributes
class Versioned(object):
def new_version(self, session):
# if on SQLA 0.6.1 or earlier,
# make sure 'id' isn't expired.
# self.id
# make us transient (removes persistent
# identity).
make_transient(self)
# set 'id' to None.
# a new PK will be generated on INSERT.
self.id = None
class VersionExtension(SessionExtension):
def before_flush(self, session, flush_context, instances):
for instance in session.dirty:
if not isinstance(instance, Versioned):
continue
if not session.is_modified(instance, passive=True):
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, extension=[VersionExtension()])
# 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')]
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