Thanks Michael.
before_flush looks like the way to go. From what I can see so far, it will
alleviate the need to call my add_or_remove function, increasing
reliability.
On Monday, 15 June 2015 10:27:06 UTC+12, Michael Bayer wrote:
>
>
>
> On 6/14/15 5:49 PM, Richard Collins wrote:
>
> I have an association object:
>
> class FolderUserAccess(db.Model):
> __tablename__ = "folderuseraccess"
> folder_id = db.Column(db.Integer, db.ForeignKey('node.id'),
> primary_key=True)
> folder = db.relationship('Folder', back_populates='access')
> user_id = db.Column(db.Integer, db.ForeignKey('user.id'),
> primary_key=True, index=True)
> user = db.relationship('User')
> access = db.Column(TINYINT, nullable=False)
> walk = db.Column(db.Boolean, nullable=False)
>
> When the object is persisted to the database, I would like to delete the
> corresponding row when access==0 and walk = False.
>
> Here is my attempt to make this happen:
>
> def add_or_remove(self):
> """Add or remove object depending on access and walk values
> ensuring no empty records are stored in database"""
>
> if self.access or self.walk:
> if self in db.session.deleted:
> db.make_transient(self)
> else:
> db.session.add(self)
> else:
> if inspect(self).persistent:
> db.session.delete(self)
>
> I call this on the object after updating it. It is complicated as it has
> to deal with the fact that somewhere else in the same transaction the
> object may have already been updated. It does not work (I will figure it
> out eventually) and strikes me as the complicated way of doing things.
>
> What I would really like to do is be able to write a custom
> implementation for when the object gets flushed. Something like:
>
> if self.access or self.walk:
> #insert or update the row
> else:
> # delete the row
>
> Is this possible? Or does anyone have any great ideas on how to achieve
> the same result in a more straightforward way than my current approach.
>
> event hooks like before_flush() or after_flush() are the best place to do
> things like this. In before_flush(), you can scan through the list of
> work to do and add new things to the Session for add() or delete(); in
> after_flush(), you can look at what's happened and then emit specific SQL
> on the session.connection() to make other changes after the fact. Other
> popular hooks include the mapper hooks before_insert(), before_update(),
> before_delete(), with these it's best to emit SQL on the given connection
> as things happen within the flush process.
>
>
>
>
>
> Thanks,
>
> Richard
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