Question

To provide an activity log in my SQLAlchemy-based app, I have a model like this:

class ActivityLog(Base):
    __tablename__ = 'activitylog'
    id = Column(Integer, primary_key=True)
    activity_by_id = Column(Integer, ForeignKey('users.id'), nullable=False)
    activity_by = relation(User, primaryjoin=activity_by_id == User.id)
    activity_at = Column(DateTime, default=datetime.utcnow, nullable=False)
    activity_type = Column(SmallInteger, nullable=False)

    target_table = Column(Unicode(20), nullable=False)
    target_id = Column(Integer, nullable=False)
    target_title = Column(Unicode(255), nullable=False)

The log contains entries for multiple tables, so I can’t use ForeignKey relations. Log entries are made like this:

doc = Document(name=u'mydoc', title=u'My Test Document',
               created_by=user, edited_by=user)
session.add(doc)
session.flush() # See note below
log = ActivityLog(activity_by=user, activity_type=ACTIVITY_ADD,
                  target_table=Document.__table__.name, target_id=doc.id,
                  target_title=doc.title)
session.add(log)

This leaves me with three problems:

1. I have to flush the session before my doc object gets an id. If I had used a ForeignKey column and a relation mapper, I could have simply called ActivityLog(target=doc) and let SQLAlchemy do the work. Is there any way to work around needing to flush by hand?

2. The target_table parameter is too verbose. I suppose I could solve this with a target property setter in ActivityLog that automatically retrieves the table name and id from a given instance.

3. Biggest of all, I’m not sure how to retrieve a model instance from the database. Given an ActivityLog instance log, calling self.session.query(log.target_table).get(log.target_id) does not work, as query() expects a model as parameter.

One workaround appears to be to use polymorphism and derive all my models from a base model which ActivityLog recognises. Something like this:

class Entity(Base):
    __tablename__ = 'entities'
    id = Column(Integer, primary_key=True)
    title = Column(Unicode(255), nullable=False)
    edited_at = Column(DateTime, onupdate=datetime.utcnow, nullable=False)
    entity_type = Column(Unicode(20), nullable=False)
    __mapper_args__ = {'polymorphic_on': entity_type}

class Document(Entity):
    __tablename__ = 'documents'
    __mapper_args__ = {'polymorphic_identity': 'document'}
    body = Column(UnicodeText, nullable=False)

class ActivityLog(Base):
    __tablename__ = 'activitylog'
    id = Column(Integer, primary_key=True)
    ...
    target_id = Column(Integer, ForeignKey('entities.id'), nullable=False)
    target = relation(Entity)

If I do this, ActivityLog(...).target will give me a Document instance when it refers to a Document, but I’m not sure it’s worth the overhead of having two tables for everything. Should I go ahead and do it this way?

Answer 1

One way to solve this is polymorphic associations. It should solve all 3 of your issues and also make database foreign key constraints work. See the polymorphic association example in SQLAlchemy source. Mike Bayer has an old blogpost that discusses this in greater detail.