I think you didn't understand the query. You would filter by the
intermediate table directly, and filter by Usuario (as you want all
Especialidad from one Usuario), yielding 2 results as it should with a
single select from one table. Right now django translates it into a more
complicated select with a join from one of the tables with the intermediary
one, which should be needed only if you are getting data from the main
table.
El jueves, 19 de noviembre de 2015, 0:04:31 (UTC-3), Josh Smeaton escribió:
>
> It might be a bit early in the day for me, but isn't that query already
> optimised? That is, it's already eliminated a join. It hasn't joined to the
> "Especialidad" table, it's only joined to the intermediate table. I *think*
> the join to the intermediate table is necessary because there could be
> duplicates.
>
> Given the tables:
>
> Usuario(pk):
> 1
> 2
>
> Intermediate(usurario_id, especialidad_id):
> 1, 1
> 1, 2
>
> Especialidad(pk)
> 1
> 2
>
> Joining Usuario to Intermediate will return 4 results in SQL (2 for each
> pk on Usuario) unless there was a distinct in there somewhere. I haven't
> tested, so I'm not sure if django does duplicate elimination, but I'm
> pretty sure it doesn't.
>
> Does this look right to you, or am I missing something?
>
> Cheers
>
>
> On Thursday, 19 November 2015 11:41:22 UTC+11, Cristiano Coelho wrote:
>>
>> 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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