Re: [sqlalchemy] Custom Flush
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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Re: [sqlalchemy] Custom Flush
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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