In the meanwhile you can directly query the intermediate model instead:
Author.books.through.objects.filter(author_id=author.id).values_list('book_id')
Le jeudi 19 novembre 2015 16:05:11 UTC-5, Cristiano Coelho a écrit :
>
> You are right. I believe an optimization like this would probably help
> just a few people, as only fetching data from the intermediary table is a
> rare thing. But if it is an easy change, which improves performance, why
> not? However I think the change is quite complicated.
>
> El jueves, 19 de noviembre de 2015, 11:51:51 (UTC-3), charettes escribió:
>>
>> Hi Cristiano,
>>
>> If I get it correctly you'd like m2m querying to start with the
>> intermediary (FROM) table and JOIN the referenced one only if more fields
>> than the primary key are selected.
>>
>> class Book(models.Model):
>> name = models.CharField(max_length=100)
>>
>> class Author(models.Model):
>> books = models.ManyToMany(Book)
>>
>> author = Author.objects.get(pk=1)
>> author.books.values_list('pk')
>>
>> Would result in the following query:
>> SELECT book_id FROM author_books WHERE author_id = 1;
>>
>> Instead of:
>> SELECT id FROM book JOIN author_books ON (book.id =
>> author_books.book_id) WHERE author_id = 1;
>>
>> I think this is a sensible optimization but I wonder about its
>> feasibility. It looks like the `pk` reference would require some special
>> handling to reference `book_id` since it's not actually a primary key on
>> the intermediate table.
>>
>> Simon
>>
>> Le mercredi 18 novembre 2015 19:41:22 UTC-5, Cristiano Coelho a écrit :
>>>
>>> Hello there,
>>>
>>> Lets say I have these two models (sorry about the spanish names!) (
>>> Django 1.8.6 and MySQL backend )
>>>
>>> class Especialidad(models.Model):
>>> nombre = models.CharField(max_length=250, blank=False, unique=True)
>>>
>>>
>>>
>>> class Usuario(AbstractBaseUser):
>>> permisosEspecialidad = models.ManyToManyField("Especialidad",
>>> blank=True)
>>>
>>> Let u be some Usuario instance, and the following query:
>>>
>>> u.permisosEspecialidad.all().values_list('pk',flat=True)
>>>
>>> The actual printed query is:
>>>
>>> SELECT `samiBackend_especialidad`.`id`
>>> FROM `samiBackend_especialidad`
>>> INNER JOIN `samiBackend_usuario_permisosEspecialidad` ON (
>>> `samiBackend_especialidad`.`id` =
>>> `samiBackend_usuario_permisosEspecialidad`.`especialidad_id` )
>>> WHERE `samiBackend_usuario_permisosEspecialidad`.`usuario_id` = 8
>>>
>>> As my understanding, since I'm only selecting the id field which is already
>>> present in the intermediary table (and is also a FK), the actual join is
>>> redundant, as I have all the info I need in this case.
>>>
>>> So the query could work like this
>>>
>>> SELECT `samiBackend_usuario_permisosEspecialidad`.`especialidad_id`
>>> FROM `samiBackend_usuario_permisosEspecialidad`
>>> WHERE `samiBackend_usuario_permisosEspecialidad`.`usuario_id` = 8
>>>
>>>
>>> I guess this works this way because this particular case might be hard to
>>> detect or won't be compatible with any additional query building, however,
>>> for ForeignKey relations, this optimization is already done (If you select
>>> the primary key from the selected model only, it wont add a join)
>>>
>>> What would be the complications to implement this? Would it worth the
>>> effort?
>>>
>>>
>>>
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