Cascade child updates onto the parent

[Colton Allen]

Hello,

I'm trying to automate a backref update.  Basically, when a child model is inserted or updated I want the parent model's "updated_at" column to mutate.  The value should be the approximate time the user-child-model was updated.  The updated_at value would not have to match the created_at/updated_at value on the child.  It would just need to mutate to a new time.

class UserModel(db.Model):
    updated_at = db.Column(db.DateTime, default=db.now, onupdate=datetime.now)


class UserChildModel(db.Model):
    user_id = db.Column(db.Integer, db.ForeignKey('user.id'), nullable=False)
    user = db.relationship('UserModel', backref='children')

user = UserModel()
save(user)
print(user.updated_at) # x

child = UserChildModel(user_id=user.id)
save(child)
print(user.updated_at) # y (value changed)

Hopefully this pseudocode is sufficient.

I'm wondering if there is an option I can specify on the orm.relationship factory.  Or will I need to define an event?

Thanks!

[Mike Bayer]

that could certainly be based on an event from the SQLAlchemy side.     a very straightforward one would be the before_insert / before_update / after_insert / after_update suite of events, I would emit an UPDATE statement against the parent table using the foreign key on the child row that's being inserted/updated.      Another approach would be a DB trigger.

the mapper level events are detailed at https://docs.sqlalchemy.org/en/13/orm/events.html?highlight=before_insert#sqlalchemy.orm.events.MapperEvents.before_insert

the "connection" right there is where you'd run your update, like:

connection.execute(update(parent).values(updated_at=datetime.now()).where(parent.id == inserted.parent_id))

Thanks!

--

SQLAlchemy -

The Python SQL Toolkit and Object Relational Mapper

 

http://www.sqlalchemy.org/

 

To post example code, please provide an MCVE: Minimal, Complete, and Verifiable Example. See http://stackoverflow.com/help/mcve for a full description.

---

You received this message because you are subscribed to the Google Groups "sqlalchemy" group.

To unsubscribe from this group and stop receiving emails from it, send an email to sqlalchemy+...@googlegroups.com.

To view this discussion on the web visit https://groups.google.com/d/msgid/sqlalchemy/490bfcd2-4ebd-4df3-98a8-516caeacbd6a%40googlegroups.com.

[Colton Allen]

Perfect.  That's exactly what I ended up doing.  I added events (after_insert/update/delete) for each backref.

For each has-many relationship (through a secondary table) I had to consider the fact that the parent model would exist in session.dirty but not trigger the "onupdate" action on the column.  So I added a generic before_update/delete event on my models' base class which is basically just target.updated_at = dt.now().

To unsubscribe from this group and stop receiving emails from it, send an email to sqlal...@googlegroups.com.