通过主键进行SQLAlchemy初始化

Question

I'm writing a sqlalchemy based python app. I want to override the sqlalchemy init method which will accept a primary key and then init it's own instance.

Something like this:

class User(Base):
    id = Column(String, primary_key=True)
    name = Column(String)

    def __init__(id):
        self = session.query(User).filter(User.id=id).first()

I know I can initialize the object using session.query, but I want to export a nice simple api that will be used by other users (It's going to be a SDK).

Any ideas? Thanks!

Answer 1

You can do this via the init method, though I would recommend against it.

There are two issues with your code snippet.

1. You forgot to include the self argument as the first argument of the __init__ method
2. Assigning to self would not work, but you can replace the dictionary of self with that of other

putting these two things together:

class User(Base):
    __tablename__ = 'users'
    id = Column(Integer, primary_key=True)
    name = Column(String)

    def __init__(self, user_id):
        res = session.query(User).filter(User.id == user_id).first()
        self.__dict__ = res.__dict__

However, I'd recommend adding a classmethod for this very specific but oft-repeated usage, ie getting an instance from the database by the primary_key

@classmethod
def get_by_id(cls, key):
    return session.query(cls).filter(cls.id == key).first()

This way it is general to all your classes that have a single column primary key.

The usage for these versions would be

u1 = User(user_id=1)
u2 = User.get_by_id(key=1)