SQLAlchemy demonstration of the "FetchedValue" feature

fetchedvalue.py

"""
This file demonstrated using the "FetchedValue" feature of SQLAlchemy for
values which are generated by the DB, without knowing *how* they are generated
in the DB.
"""
from sqlalchemy import Column, DateTime, FetchedValue, Unicode, create_engine
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import sessionmaker

Base = declarative_base()

class Foo(Base):
    __tablename__ = "foo"

    # mapper-args are necessary to tell SA to use the "RETURNING" statements.
    # Note that this will cause overhead on DBs which don't support "RETURNING"
    __mapper_args__ = {"eager_defaults": True}

    name = Column(Unicode, primary_key=True)

    # Assuming the server assigns a value by default, using "FetchedValue"
    # marks this column as a column that should be added in a "RETURNING"
    # statement. On DBs not supporting "RETURNING", this will cause an
    # additional SELECT
    created = Column(DateTime, server_default=FetchedValue())


engine = create_engine('postgresql://malbert@/malbert', echo=True)

Session = sessionmaker(bind=engine)
session = Session()
instance = Foo(name="foo")
session.add(instance)
session.flush()  # Flush is needed. FetchedValue does not trigger autoflush
print('>>---', instance.created)