Hi Patryk,
> Currently we have Prefetch objects that kind of solve this problem for M2M
> relations but perhaps it would be nice to also be able to use QuerySets in
> select_related() or even in filter(). I don't think Prefetch objects are
best
> suited for that and I personally find having to instantiate them
explicitly
> kind of ugly.
>From what I understand you're not a fan of Prefetch objects but I think most
of your problems can be solved by using them and custom managers as the
formers
also support foreign keys[1].
class AuthorDisplayManager(models.Manager):
def get_queryset(self):
queryset = super().get_queryset()
return queryset.filter(
visible=True,
).annotate(
num_books=Count('books'),
num_awards=Count('awards'),
).prefetch_related(
'foo',
)
class Author(models.Model):
...
for_display = AuthorDisplayManager()
class BookDisplayManager(models.Manager):
def get_queryset(self):
queryset = super().get_queryset()
return queryset.filter(
visible=True,
).prefetch_related(
Prefetch(
'author',
Author.for_display.all(),
)
)
class Book(models.Model):
...
for_display = BookDisplayManager()
Simon
[1] https://code.djangoproject.com/ticket/17003
Le vendredi 22 janvier 2016 05:25:24 UTC-5, Patryk Zawadzki a écrit :
>
> Hi,
>
> Currently the way QuerySets work makes it hard to reuse code. Consider an
> example where I have template tags to render an author and a book (that in
> turn displays author). I don't want my template code to execute any
> additional queries per row displayed so I am forced to prefetch everything
> upfront. Now if I want to display a list of authors and a list of books my
> code is vastly different:
>
> ```
> authors = Author.objects.filter(visible=True)
> authors_for_display = authors.annotate(num_books=Count('books'),
> num_awards=Count('awards')).prefetch_related('foo')
> ```
>
> vs.
>
> ```
> books = Book.objects.filter(visible=True, author__visible=True)
> books_for_display =
> books.annotate(author_num_books=Count('author__books'),
> author_num_awards=Count('author__awards')).select_related('author').prefetch_related('author__foo')
> ```
>
> Both cases need to do the exact same filtering, annotations and prefetches
> for the Author object but notation is so different that no code reuse can
> happen ("baz" vs. "bar__baz" vs. "foo__bar__baz"). The behaviour of
> annotations is also different as now they end up on the Book object which
> forces me to write some glue code. If my structure gains another level of
> depth I will be forced to go to even greater extents.
>
> Now consider that I have many other objects that reference the Author and
> thus I have many places in code that implement the same logic but each time
> with a slight syntactic twist. What if a front-end engineer asks me to
> change the information fetched about the Author? Or perhaps I discover that
> half of the query time is spent fetching a megabyte of Author's extended
> biography in HTML that the front-end never uses outside of that Author's
> page. I am forced to hunt down and update all of these.
>
> Currently we have Prefetch objects that kind of solve this problem for M2M
> relations but perhaps it would be nice to also be able to use QuerySets in
> select_related() or even in filter(). I don't think Prefetch objects are
> best suited for that and I personally find having to instantiate them
> explicitly kind of ugly. To me having another SelectRelated object is
> probably a no-go.
>
> Ideas?
>
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