This sounds like a reasonable request, I don't yet have an opinion on API
or anything. One tiny thing I'd like to note though, "change DeferredAttribute
to find all *other* DeferredAttributes". I don't think `finding` is the
right way to think about it, a `DeferredAttribute` with loadall semantics
should know about the specific set of objects that came from the queryset.
Alex
On Thu, Apr 25, 2013 at 12:06 PM, Adrian Holovaty <adr...@holovaty.com>wrote:
> At the moment, if you call defer() or only() on a QuerySet, then access
> the deferred fields individually, *each* access of a deferred field will
> result in a separate query.
>
> For example, assuming a User model with username/bio/location fields, this
> is what currently happens:
>
> """
> >>> u = User.objects.only('username').get(id=1)
> # Results in: SELECT id, username FROM users WHERE id=1
>
> >>> u.bio
> # Results in SELECT bio FROM users WHERE id=1
>
> >>> u.location
> # Results in SELECT location FROM users WHERE id=1
> """
>
> I'd like there to be a way of retrieving *all* deferred fields the first
> time a deferred field is accessed. So with the above example, this would
> happen instead:
>
> """
> >>> u = User.objects.only('username').get(id=1)
> # Results in: SELECT id, username FROM users WHERE id=1
>
> >>> u.bio
> # Results in SELECT bio, location FROM users WHERE id=1
>
> >>> u.location
> # No database query
> """
>
> Here's my use case. In my app, I'm storing frequently accessed user
> attributes (id, username, etc.) in signed cookies, to prevent an
> unnecessary database hit on every page view. I create a User object with
> those attributes, so that I can take advantage of various methods on the
> User class, but I want the other User fields to be lazily loaded. I have
> this all working except for the last piece, which is to change lazy loading
> such that it loads *everything* else the first time a deferred field is
> accessed.
>
> So that's the "what" and "why" -- here's the how...
>
> The current implementation is in django/db/models/query_utils.py -- see
> the DeferredAttribute class. When you call only() or defer() on a QuerySet,
> the resulting model instance will have a DeferredAttribute instance for
> each deferred field. The problem is that DeferredAttributes only load their
> own column's data, not the data for all *other* DeferredAttributes on the
> given model instance. Hence, the individual SQL queries for each column.
>
> To solve this, we would need to change DeferredAttribute to find all
> *other* DeferredAttributes on the given model and load them in a single
> query somehow.
>
> Also, I should mention that this should be *optional* behavior, as the
> current behavior is reasonable for the common case. The API for specifying
> this "load everything" behavior is a separate discussion. Perhaps a keyword
> argument like: User.objects.only('username', loadall=True).
>
> Thoughts?
>
> Adrian
>
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