SQLAlchemy中一行的创建时间戳

Question

Using SQLAlchemy ORM, what is a good way to record the creation time of a record, without overwriting the creation time on subsequent updates?

Here's a sample table definition:

class Car:
    model = Column(String, primary_key=True)
    latest_model_year = Column(Integer)
    created = Column(DateTime)

I would like to use this code to add/update a Car row in the database:

car = Car(
    model = 'Accord',
    latest_model_year = 2022,
)
session.merge(car)

... but that doesn't set the created column when a new row is inserted.

I could do something like this:

if session.query(Car).filter(Car.model=='Accord').first():
    # this car already exists, so update the existing row
    car = Car(
        model = 'Accord',
        latest_model_year = 2022,
    )
    session.merge(car)
else:
    # this car does not exist, so create a new row
    car = Car(
        model = 'Accord',
        latest_model_year = 2022,
        created = datetime.now(),
    )
    session.add(car)

... but that's a lot of repetitive code. Is there a better way?

Answer 1

Try:

from sqlalchemy.sql import func

class Car:
    model = Column(String, primary_key=True)
    latest_model_year = Column(Integer)
    created = Column(DateTime, default=func.now())  # <- HERE

Documentation: Client-Invoked SQL Expressions