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):
    …