inspect a scalar relationship property when it is loaded

[Kent]

@validates and 'set' attribute events will only fire when the user sets a property (setattr), not when initially loaded by the orm.

Is there a way to intercept (for inspection) a scalar relationship property instance when it is loaded?  I don't think the 'load' event will work because I won't know if the current instance is being loaded in the context of setting an instrumented attribute.

For example:

If I have a parent object, I want to know when parent.childobj is populated by the orm framework (whether on a lazy load or joined load).

Is that possible?

Thanks in advance!

[mike bayer]

On 10/28/2016 10:48 AM, Kent wrote:
> @validates and 'set' attribute events will only fire when the /user/

> sets a property (setattr), not when initially loaded by the orm.
>
> Is there a way to intercept (for inspection) a scalar relationship
> property instance when it is loaded? I don't think the 'load' event
> will work because I won't know if the current instance is being loaded
> in the context of setting an instrumented attribute.
>
> For example:
>
> If I have a parent object, I want to know when parent.childobj is
> populated by the orm framework (whether on a lazy load or joined load).
>
> Is that possible?
>
> Thanks in advance!

based on the laws of twos, being that any random issue that has never
been asked ever for ten years will suddenly be asked for twice within
two days, the answer is no! the event proposed would be
collection_populate, at the same time we'd add scalar_populate, see
https://groups.google.com/d/msg/sqlalchemy/IfuUwUgKwW0/5ZgPmpAvBAAJ .

Interim solution is to subclass Query and override __iter__. The
load() event will get you eagerly-loaded attributes though, set it on
"parent"'s class and then inspect "childobj".




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[Kent Bower]

The load() event would also capture if it were subqueryload()ed, right?

And overriding Query.__iter__ wouldn't catch the cases where the scalar is a "use_get" lookup that was already in session.identity_map...  No good way to intercept attributes.set_committed() for that, is there?

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[mike bayer]

On 10/28/2016 11:55 AM, Kent Bower wrote:
> The load() event would also capture if it were subqueryload()ed, right?

any eagerload.

>
> And overriding Query.__iter__ wouldn't catch the cases where the scalar
> is a "use_get" lookup that was already in session.identity_map...

right, would need to catch that too.

No
> good way to intercept attributes.set_committed() for that, is there?

there's not events for that because setting attribute values for loads
is usually done using more direct paths than that and speed is extremely
critical. that's why there's not a lot of "load" events. new events
that are local to relationship loading are feasible though.

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