#35810: Provide `Select` class for `select_related` (like `Prefetch` exists for
`prefetch_related`)
-------------------------------------+-------------------------------------
Reporter: Bart van Andel | Type: New
| feature
Status: new | Component: Database
| layer (models, ORM)
Version: 5.1 | Severity: Normal
Keywords: query optimization | Triage Stage:
| Unreviewed
Has patch: 0 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
-------------------------------------+-------------------------------------
Use case:
Given a model `Chat` with many linked `Message`s:
{{{
class Chat(models.Model):
created_at = models.DateTimeField(auto_now_add=True)
users = models.ManyToManyField(User)
def last_message(self):
return self.jobmatch_set.latest("created_at")
def last_message_using_prefetched_set(self):
return max(self.message_set.all(), key=lambda item:
item.created_at, default=None)
def last_message_using_prefetched_single_item_sets(self):
assert hasattr(self, "_last_message_set"), "_last_message_set not
populated":
return self._last_message_set[0] if len(self._last_message_set) >
0 else None
class Message(models.Model):
created_at = models.DateTimeField(auto_now_add=True)
chat = models.ForeignKey(Chat)
}}}
We'd like to load a list of chats with the latest message of each chat.
This doesn't seem to be easily achievable at the moment. Example:
{{{
# Valid, but causes an additional trip for each chat:
chats = Chat.objects.all()
last_messages = [chat.last_message for chat in chats]
# Invalid:
chats = Chat.objects.select_related("last_message")
# Also invalid:
chats = Chat.objects.prefetch_related("last_message")
# Valid approach (2 queries), which may however load a huuuuge amount of
data, so in general this should be avoided
# Also, there is no guarantee that the prefetched set is not filtered from
the perspective of the last message getter property
chats = Chat.objects.prefetch_related("message_set")
last_messages = [chat.last_message_using_prefetched_set for chat in chats]
# Workaround (2 queries) using prefetch to get the single latest (if
exist) message per chat:
chats = Chat.objects.prefetch_related(
Prefetch(
"chat_set",
queryset=Chat.objects.order_by("-created_at")[:1],
to_attr="_last_match_set",
),
)
last_messages = [chat.last_message_using_prefetched_single_item_sets for
chat in chats]
}}}
I do appreciate that there are probably very good reasons why the invalid
calls in the example above won't be able to work, unless maybe with some
still-to-be-invented annotations.
Now, the `Prefetch` class was introduced quite recently to even make the
above possible. It can only be used to prefetch _sets of items_ though,
not _single values_, so using something like `.latest("created_at")` is
out of the question.
Could something like that also be implemented to support this syntax?
Basically the same effect as `Prefetch`, except with a single output (or
None).
{{{
chats = Chat.objects.select_related(
Select(
"chat_set",
queryset=Chat.objects.latest("created_at"),
to_attr="last_match",
),
)
}}}
Note: I couldn't find a similar question in the existing ticket list; if I
missed something, I do apologize.
--
Ticket URL: <https://code.djangoproject.com/ticket/35810>
Django <https://code.djangoproject.com/>
The Web framework for perfectionists with deadlines.
--
You received this message because you are subscribed to the Google Groups
"Django updates" group.
To unsubscribe from this group and stop receiving emails from it, send an email
to [email protected].
To view this discussion on the web visit
https://groups.google.com/d/msgid/django-updates/01070192533b8e69-6d479dd9-bb87-4132-b585-2ec07cf2a5c8-000000%40eu-central-1.amazonses.com.
