works for me! at least I have the plan for the next time this comes up.
On 10/27/2016 12:42 PM, Paul Winkler wrote:
Wow, thanks for the very detailed reply Mike!
This is rather anticlimactic followup I'm afraid :)
But we mulled this over a bit and just slapped an index on the column in
question instead.
- Paul
On Wednesday, October 26, 2016 at 9:19:23 PM UTC-4, Mike Bayer wrote:
On 10/26/2016 05:50 PM, Paul Winkler wrote:
> So I currently have this:
>
> thingies = relationship(
> "Thingy",
> order_by="Thingy.sort_order",
> collection_class=ordering_list("sort_order"),
> )
>
>
> But could maybe change it to:
>
>
> thingies = relationship(
> "Thingy",
> collection_class=ordering_list(
> "sort_order", reorder_on_append=True)
> )
>
> I *think* that would do what I want, and the related items would
be put
> in place by reordering the list as each is appended during load,
> but a) that seems like a rather inefficient sort algorithm (I'd
rather
> load the full list
> and then implicitly call reorder() exactly once, but I don't see a
hook
> for doing something
> like that on object load) and b) `reorder_on_append` comes with big
> warnings about
> dangers of concurrent modification.
ordering list has lots of weird edges to it that I've never had the
means to figure out the solution towards. In this case, sorting by
one-append-at-a-time isn't that efficient, but otherwise I don't think
you'd have the concurrent modification issue here, I'm pretty sure
ordering list is meant to work with the ORM's normal append to the
collection as the source. I think the edges have more to do with
when
the sort key is the primary key and people are trying to change the
sort
keys and stuff like that.
But the idea to hit the list after population and sort it just once,
and
not really worrying about ordering list being involved at all,
is a good one. Looking around though, we have a lot of ways to
get a
hold of an object event for when we first build it from a row, but,
there's no hook that corresponds to, "we've eagerly loaded all the
collections and we're about to yield the object", nor is there a "we've
just lazy loaded this collection and stuffed it all into the list"
event. These are events we probably should have, and it might be good
as a "collection_loaded" event that works equally across
lazy/eager/whatever loads, and is oriented towards "I want to mutate
the
collection in a certain way", which I think right now we have via the
attributes.set_committed_value method (you'd just set a new list
into it
with the ordering you want).
right now, without that "collection_loaded" event, to do this approach
you'd be stuck subclassing Query to intercept results as they are
emitted via __iter__. You'd only have to override the __iter__ method
though with a filter and it can be programmatically determined what
should be done with the objects. It would be either: 1. detect all the
objects in the result that are of type Foo and have a collection ".bar"
that you know was eagerly loaded and needs to be sorted, or 2. detect
that we just lazy loaded the .bar collection on a Foo, sort the result
(the event hook would be applied to "Foo.bar" in the general sense and
fire off for any "here's a newly populated Foo.bar for you to mutate in
place").
We could even write an interim Query subclass that overrides __iter__
using this technique and then produces the "collection loaded" event
itself. You could in theory code to that, we could write tests for
it,
then when SQLAlchemy adds the actual "collection_loaded" event hook
you're all set.
Of course this all sounds like I'm looking for free contributions to my
software, and you'd be right :). TLDR, sort of doable-ish right now
with ugliness, should be doable nicely with a new API feature, and a
path can probably be drawn between those points as well so that the
feature can live first as an external hack and later as a supported
API.
>
> I could of course wrap the relationship in a property that does
the sorting,
> but then would have to update a bunch of existing query code that
assumes
> this attribute is a relationship that can be joined on.
right....well ....right there's hooks that could automate that too (in
fact the "attribute_instrument" event can probably used to automate
this) but then, you're still having to guess if you sorted the
collection already or not, as well as making a custom descriptor
here is
pretty awkward, not that straightforward. the
intercept-the-collection-load event seems a lot nicer.
>
> I feel like I'm barking up the wrong tree. Any pointers on where I
> should be looking?
>
> - Paul
>
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