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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