Question

I’m trying to share a simple functionality across all my models (timestamping) and am going with the “augmenting the base” approach as described in SQLA docs. So far I did this:

import sqlalchemy as sa
from datetime import datetime as dt

class EntityBase(object):

    @declared_attr
    def __tablename__(cls):
        return cls.__name__.lower()

    id = sa.Column(sa.Integer, primary_key=True)
    last_update = sa.Column(sa.DateTime, default=dt.utcnow())

def update_entity(mapper, connection, target):
    target.last_update = dt.utcnow()


Entity = declarative_base(cls=EntityBase)

sa.event.listen(Entity, 'before_insert', update_entity)
sa.event.listen(Entity, 'before_update', update_entity)

All my models are derived from the Entity class. But at runtime I get sqlalchemy.orm.exc.UnmappedClassError: Class 'sqlalchemy.ext.declarative.Base' is not mapped. What am I doing wrong?

UPDATE
I’ve circumvented the problem by doing simple preprocessing, like this:

def setupEntities():
    ...
    for cls in Entity.__subclasses__():
        listen(cls, 'before_insert', update_entity)
        listen(cls, 'before_update', update_entity)
    ...

…but I’d like to hear about a right way to do it.

Answer 1

interesting here is that the way you did it above probably should work. It would be handy. So I’ve added a ticket for that http://www.sqlalchemy.org/trac/ticket/2585 .

For now, a way you can do this is to set an event listener for new mappings along with your Base:

Entity = declarative_base(cls=EntityBase):

from sqlalchemy.orm import mapper
@event.listens_for(mapper, 'mapper_configured')
def set_events(mapper, class_):
   if issubclass(class_, Entity):
        listen(class_, 'before_update', update_entity)
        listen(class_, 'before_insert', update_entity)

in fact if I implement the feature for #2585 I’d probably have to do it very similarly to this also.