Hooking into sqlalchemy models

Question

I'm looking to hook into the model creation cycle for sqlalchemy models. For example on create or on save (like in the Ruby ORM ActiveRecord, in fact I'm moving a model from ActiveRecord to SqlAlchemy).

Events looks like what I need: http://docs.sqlalchemy.org/en/rel_0_7/core/event.html, but I haven't found more detailed examples, yet. I'd like to hear someone's experience with this.

Are there similar facilities in sqlalchemy for doing things with a model/instance based on certain cues, e.g. after_create?

Answer 1

Events are pretty simple once you get the hang of it. Here is a quick example using events

import uuid    

from sqlalchemy.event import listen

from mypackage.models import Base


def generate_license(mapper, connect, target):
    target.generate_license()

class User(Base):
    __tablename__ = "users"
    id = Column(String(36))
    license = Column(String(256))

    def generate_license(self):
        if not self.license:
            self.license = str(uuid.uuid4())
        return self.license

listen(User, 'before_insert', generate_license)

Alternately, you can use decorators:

from sqlalchemy.event import listens_for
…
class User(Base):
    …

@listens_for(User, 'before_insert')
def generate_license(mapper, connect, self):
    …

Answer 2

from sqlalchemy.event import listen_for
…
class User(Base):
…
    @listen_for(User, 'before_insert')
    @staticmethod
    def generate_license(mapper, connect, self):
…

This will return

NameError: name 'User' is not defined