Sorry, I will clarify what I was asking. If I had my class example from
above:
class A(Base):
foo = Column(String)
bar = Column(String)
foo_updated = Column(DateTime, onupdate=update_fn) # Should only update
when foo is updated
Based on what information is in the context, sometimes I may not want to
update the value. However, my understanding is that onupdate_fn *must*
return a value always.
Given that we need to return a value from onupdate_fn, one idea I had was
to return a value to emit:
UPDATE A SET bar="new value", foo_updated=A.foo_updated WHERE id = 1
If we can somehow specify "set foo_updated to whatever value it already is"
in SQL land, then we should be set! However, I tried some variations of
this:
- `return 'foo_updated'`
- `return '"foo_updated"'
- `return A.foo_updated`
- `return A.__table__.c.foo_updated`
and none of these ended up working. Do you know if there's a way to specify
an onupdate function to return a SQL column?
On Thursday, May 16, 2019 at 2:49:56 PM UTC-7, Mike Bayer wrote:
>
> On Thu, May 16, 2019 at 4:26 PM Tony Cao <[email protected]
> <javascript:>> wrote:
> >
> > Ohh I see thanks for the help!
> >
> > And just to confirm on the onupdate front, is it not possible to return
> the current value of the object being updated? It's ok to emit an
> extraneous update if the value doesn't change.
>
> inside the onupdate handler you can see the current statement and the
> parameters in the context, as far as the "a_1" thing, the parameters
> for the columns being SET should be the same keys as the column
> itself, it's the WHERE clause where you'd see the underscore thing
> happening. show me what you get if you want to work with that.
>
>
>
>
> >
> > On Wednesday, May 15, 2019 at 5:30:06 PM UTC-7, Mike Bayer wrote:
> >>
> >> On Wed, May 15, 2019 at 6:10 PM Tony Cao <[email protected]> wrote:
> >> >
> >> > I mean query.update().
> >> >
> >> > Ah our goal was to make it so the update in question happened
> automatically without the developer having to explicitly specify it - in
> that case both obj.attr = x and obj_class.query.update({obj.attr: x})
> should both trigger an update to obj.attr_modified. That's why I initially
> asked about onupdate, as it is triggered for both cases.
> >>
> >>
> >> for query.update() you can use the before_compile_update() and modify
> >> the context.values dictionary in place to affect the UPDATE statement
> >> generated by query.update(). See
> >>
> https://docs.sqlalchemy.org/en/13/orm/events.html?highlight=before_compile_update#sqlalchemy.orm.events.QueryEvents.before_compile_update.
>
>
> >> Additional detail regarding .values will be up within 30 minutes.
> >>
> >>
> >> >
> >> >
> >> > On Wednesday, May 15, 2019 at 2:38:19 PM UTC-7, Mike Bayer wrote:
> >> >>
> >> >> Bulk query updates, you mean, query.update() ? Or
> >> >> session.bulk_update_mappings() ? In both cases you are
> >> >> programatically providing the VALUES clause, so you know from your
> own
> >> >> data what the UPDATE statement will be.
> >> >>
> >> >>
> >> >>
> >> >> On Wed, May 15, 2019 at 4:14 PM Tony Cao <[email protected]>
> wrote:
> >> >> >
> >> >> > Ah but it also looks like the before_update event isn't triggered
> when doing bulk query updates, which we'd like to also update on. Is there
> a way to track those?
> >> >> >
> >> >> > On Wednesday, May 15, 2019 at 10:38:24 AM UTC-7, Mike Bayer wrote:
> >> >> >>
> >> >> >> you can inspect() the object and look at
> inspect(obj).attrs['some_attr'].history
> >> >> >>
> >> >> >>
> https://docs.sqlalchemy.org/en/13/orm/internals.html#sqlalchemy.orm.state.AttributeState
>
> >> >> >>
> >> >> >>
> >> >> >> On Wed, May 15, 2019 at 1:14 PM Tony Cao <[email protected]>
> wrote:
> >> >> >> >
> >> >> >> > Hi,
> >> >> >> >
> >> >> >> > Thanks for the response! I also tried looking into
> before_update, but is there a recommended way to figure out what columns
> are being updated from that event? I had found this post (
> https://stackoverflow.com/questions/15642286/how-can-i-get-a-sqlalchemy-orm-objects-previous-state-after-a-db-update)
>
> that suggests looking at the history of the attribute state - is there a
> better way?
> >> >> >> >
> >> >> >> > On Wednesday, May 15, 2019 at 7:55:44 AM UTC-7, Mike Bayer
> wrote:
> >> >> >> >>
> >> >> >> >> On Mon, May 13, 2019 at 4:18 PM Tony Cao <[email protected]>
> wrote:
> >> >> >> >> >
> >> >> >> >> > Hi all,
> >> >> >> >> >
> >> >> >> >> > Is there way to use Column.onupdate conditionally? For
> example, say I have:
> >> >> >> >> >
> >> >> >> >> > class A(Base):
> >> >> >> >> > foo = Column(String)
> >> >> >> >> > bar = Column(String)
> >> >> >> >> > foo_updated = Column(DateTime, onupdate=update_fn) #
> Should only update when foo is updated
> >> >> >> >> >
> >> >> >> >> > def update_fn(context):
> >> >> >> >> > if ...: # How can I check if only foo was updated?
> >> >> >> >> > return datetime.now()
> >> >> >> >> > else:
> >> >> >> >> > return ... # How can I say to not update?
> >> >> >> >> >
> >> >> >> >> > Is there a way to define update_fn to only update
> foo_updated when foo changes? I can look at
> context.get_current_parameters() to see what columns are being used in the
> compiled statement, but it doesn't explicitly say which columns are the
> ones actually being updated; for example, if I have
> >> >> >> >> >
> >> >> >> >> > A.query.filter(A.bar == 'test').update({A.foo: 'new'},
> synchronize_session=False)
> >> >> >> >> >
> >> >> >> >> > then context.get_current_parameters will return a dict with
> keys for both 'bar' and 'foo', although it looks like it suffixes the
> filter param with a '_1' - is that something I can rely on to know if a
> column is used as a filter instead of an update? And beyond that, is there
> a way I can specify to not update the column?
> >> >> >> >> >
> >> >> >> >> > Alternatively, is there another approach recommended to
> doing this?
> >> >> >> >>
> >> >> >> >> the most common way is to use the ORM level before_update
> event:
> >> >> >> >>
> >> >> >> >>
> https://docs.sqlalchemy.org/en/13/orm/events.html?highlight=before_update#sqlalchemy.orm.events.MapperEvents.before_update
>
> >> >> >> >>
> >> >> >> >> there's also before_execute() at the Core level, but before
> the UPDATE
> >> >> >> >> statement is even written, e.g. at the ORM level, is the best
> way.
> >> >> >> >> Once you are in the onupdate Python function, you have to
> return a
> >> >> >> >> value, as this occurs well after the UPDATE statement has been
> >> >> >> >> compiled.
> >> >> >> >>
> >> >> >> >>
> >> >> >> >>
> >> >> >> >>
> >> >> >> >> >
> >> >> >> >> > 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.
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>
>
> >> >> >> >> > For more options, visit https://groups.google.com/d/optout.
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> >> >> >> > --
> >> >> >> > SQLAlchemy -
> >> >> >> > The Python SQL Toolkit and Object Relational Mapper
> >> >> >> >
> >> >> >> > http://www.sqlalchemy.org/
> >> >> >> >
> >> >> >> > To post example code, please provide an MCVE: Minimal,
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> >> >> > --
> >> >> > SQLAlchemy -
> >> >> > The Python SQL Toolkit and Object Relational Mapper
> >> >> >
> >> >> > http://www.sqlalchemy.org/
> >> >> >
> >> >> > To post example code, please provide an MCVE: Minimal, Complete,
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> >> > The Python SQL Toolkit and Object Relational Mapper
> >> >
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> > The Python SQL Toolkit and Object Relational Mapper
> >
> > http://www.sqlalchemy.org/
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The Python SQL Toolkit and Object Relational Mapper
http://www.sqlalchemy.org/
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