SQLAlchemy: replacing object with a new one, following defaults - sqlalchemy

Question

I want to create a new instance of an SQLAlchemy object, so that fields are filled with default values, but I want to commit that to the database generating an UPDATE to a row that already exists with the same primary key, effectively resetting it to the default values. Is there any simple way to do that?

Answer 1

I have tried to do that and failed, because SQLAlchemy session tracks state of objects. So there is no easy way to make session to track new object as persistent one.
But you want to reset object to default, do you? There is a simple way to do that:
from sqlalchemy.ext.declarative import declarative_base
class Base(object):
def reset(self):
for name, column in self.__class__.__table__.columns.items():
if column.default is not None:
setattr(self, name, column.default.execute())
Base = declarative_base(bind=engine, cls=Base)
This adds reset method to all your model classes.
Here is the complete working example to fiddle with:
import os
from datetime import datetime
from sqlalchemy import create_engine
from sqlalchemy import Column, Integer, String, DateTime
from sqlalchemy.orm import sessionmaker
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.sql import functions
here = os.path.abspath(os.path.dirname(__file__))
engine = create_engine('sqlite:///%s/db.sqlite' % here, echo=True)
Session = sessionmaker(bind=engine)
class Base(object):
def reset(self):
for name, column in self.__class__.__table__.columns.items():
if column.default is not None:
setattr(self, name, column.default.execute())
Base = declarative_base(bind=engine, cls=Base)
class Thing(Base):
__tablename__ = 'things'
id = Column(Integer, primary_key=True)
value = Column(String(255), default='default')
ts1 = Column(DateTime, default=datetime.now)
ts2 = Column(DateTime, default=functions.now())
def __repr__(self):
return '<Thing(id={0.id!r}, value={0.value!r}, ' \
'ts1={0.ts1!r}, ts2={0.ts2!r})>'.format(self)
if __name__ == '__main__':
Base.metadata.drop_all()
Base.metadata.create_all()
print("---------------------------------------")
print("Create a new thing")
print("---------------------------------------")
session = Session()
thing = Thing(
value='some value',
ts1=datetime(2014, 1, 1),
ts2=datetime(2014, 2, 2),
)
session.add(thing)
session.commit()
session.close()
print("---------------------------------------")
print("Quering it from DB")
print("---------------------------------------")
session = Session()
thing = session.query(Thing).filter(Thing.id == 1).one()
print(thing)
session.close()
print("---------------------------------------")
print("Reset it to default")
print("---------------------------------------")
session = Session()
thing = session.query(Thing).filter(Thing.id == 1).one()
thing.reset()
session.commit()
session.close()
print("---------------------------------------")
print("Quering it from DB")
print("---------------------------------------")
session = Session()
thing = session.query(Thing).filter(Thing.id == 1).one()
print(thing)
session.close()

Answer 2

Is there any simple way to do that?
Upon further consideration, not really. The cleanest way will be to define your defaults in __init__. The constructor is never called when fetching objects from the DB, so it's perfectly safe. You can also use backend functions such as current_timestamp().
class MyObject(Base):
id = Column(sa.Integer, primary_key=True)
column1 = Column(sa.String)
column2 = Column(sa.Integer)
columnN = Column(sa.String)
updated = Column(sa.DateTime)
def __init__(self, **kwargs):
kwargs.setdefault('column1', 'default value')
kwargs.setdefault('column2', 123)
kwargs.setdefault('columnN', None)
kwargs.setdefault('updated', sa.func.current_timestamp())
super(MyObject, self).__init__(**kwargs)
default_obj = MyObject()
default_obj.id = old_id
session.merge(default_obj)
session.commit()