Michael Bayer wrote:
Specifically, the following:
- change the primary key of the instance by deleting it so that the dataabse
assigns a new primary key?
that wont work. you need to take the object and call make_transient() on it, re-add it to the session.
Where is make_transient documented? (failing that, where do I import it
from?)
> the latest you could do this would be sessionextension.before_flush().
How do I know what instances I need to worry about in before_flush?
http://www.sqlalchemy.org/docs/05/reference/orm/interfaces.html#sqlalchemy.orm.interfaces.SessionExtension.before_flush
implies is `instances` list passed can't be relied upon...
- do a query to update an existing row? (where would I get the session from?)
you can do that, if you use the given connection. session is from
object_session(instance).
So the session returned by object_session will always be connected to
the connection provided in the `connection` parameter?
- change the row the instance corresponds to and somewhow return a "new replacement
instance"?
not sure what that means
That's this bit from my previous example:
>>> session.add(x)
>>> session.commit()
>>> x.id, x.name, x.value, x.valid_from, x.valid_to
1, 'foo', 100, 2010-06-29 09:00, None
>>> x.value = 200
>>> session.commit()
>>> x.id, x.name, x.value, x.valid_from, x.valid_to
2, 'foo', 200, 2010-06-29 09:05, None
>>> for thing in session.query(MyThing).all():
... print x.id, x.name, x.value, x.valid_from, x.valid_to
1, 'foo', 100, 2010-06-29 09:00, 2010-06-29 09:05
2, 'foo', 200, 2010-06-29 09:05, None
`x` used to correspond to the row with id of 1, but now corresponds to
the row with the id of 2. That's the effect I'm looking to achieve.
so far every versioning thing I've written is more appropriate for
before_flush() rather than before_insert/update.. its likely easier to keep
thing there since you have no restrictions.
Are there good examples of before_flush() implementations to look at
anywhere?
> Also, feel free to use another recipe for versioning based on the
model you'd like. I'm using a recipe right now that has a "primary"
object which points to several "temporal" objects. When you say
parent.some_child.foo = 'bar', the before_flush() extension checks for
real changes using session.is_modified(),
The model we're looking for is roughly:
- only primary objects in one table
- the valid_from and valid_to form a linked list of version validity
- only certain column changes cause a new version to be created (eg: if
the 'notes' column changes, we don't want to create a new version; would
session.is_modified() still help with that?)
then uses the aforementioned make_transient() to convert the UPDATE into an
INSERT.
okay, so we would need to cause an update or the original row, an insert
of the new row and then mutate the object to be attached to the new row,
and no longer attached to the updated row, if that makes sense?
Any ideas gratefully received! :-)
cheers,
Chris
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