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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