Selecting multiple FilteredRelation's returns only the last one.

[Morteza PRK]

When using FilteredRelation multiple times in single query, results are wrong.
consider following example (with Django 2.2.6):

The models are:

from django.db import models


class UserType(models.Model):
    pass


class Commodity(models.Model):
    pass


class CommodityCount(models.Model):
    commodity = models.ForeignKey(Commodity, on_delete=models.CASCADE)
    user_type = models.ForeignKey(UserType, on_delete=models.CASCADE)
    small_box = models.IntegerField(default=0)

    class Meta:
        unique_together = ('commodity', 'user_type')


class CommodityDiscount(models.Model):
    commodity = models.ForeignKey(Commodity, on_delete=models.CASCADE)
    user_type = models.ForeignKey(UserType, on_delete=models.CASCADE)
    small_box = models.IntegerField(default=0)

    class Meta:
        unique_together = ('commodity', 'user_type')

following query is what I want:

query = Commodity.objects.annotate(
    cnt=FilteredRelation('commoditycount', condition=Q(commoditycount__user_type_id=1)), 
    dst=FilteredRelation('commoditydiscount', condition=Q(commoditydiscount__user_type_id=1))
).filter(
    cnt__isnull=False, 
    dst__isnull=False
).select_related('cnt','dst')

resulting query is ok:

SELECT
    "ma_commodity"."id",
    cnt."id",
    cnt."commodity_id",
    cnt."user_type_id",
    cnt."small_box",
    dst."id",
    dst."commodity_id",
    dst."user_type_id",
    dst."small_box"
FROM
    "ma_commodity"
    INNER JOIN "ma_commoditycount" cnt ON (
        "ma_commodity"."id" = cnt."commodity_id"
        AND (cnt."user_type_id" = 1)
    )
    INNER JOIN "ma_commoditydiscount" dst ON (
        "ma_commodity"."id" = dst."commodity_id"
        AND (dst."user_type_id" = 1)
    )
WHERE
    (
        cnt."id" IS NOT NULL
        AND dst."id" IS NOT NULL
    )

but results are not ok:

first_item = query.first()
print(dir(first_item))
>>> [ ... '_state', 'check', 'clean', 'clean_fields', 'commoditycount_set', 'commoditydiscount_set', 
    'date_error_message', 'delete', 'dst', 'from_db', 'full_clean', 'get_deferred_fields', 'id', 'objects', 
    'pk', 'prepare_database_save', 'refresh_from_db', 'save', 'save_base', 'serializable_value', 'unique_error_message', 'validate_unique'
    ]

as you can see, there is dst, but no cnt.
if I remove dst and its related FilteredRelation, cnt re appear.

[Mariusz Felisiak]

Thanks for this report. I attached a simple test.

Reproduced at dafdfd6a60638c4edcca7c4e65d11c0af654d759.

[Mariusz Felisiak]

Test.

[Mariusz Felisiak <felisiak.mariusz@…>]

In e1ae2b0:

Refs #30854 -- Moved local_setter() outside the loop in SQLCompiler.get_related_selections().

[Mariusz Felisiak <felisiak.mariusz@…>]

In 6a75cea7:

Fixed #30854 -- Fixed QuerySet.select_related() with multiple FilteredRelations.

[Mariusz Felisiak <felisiak.mariusz@…>]

In c1cfec6b:

[3.0.x] Fixed #30854 -- Fixed QuerySet.select_related() with multiple FilteredRelations.

Backport of 6a75cea76a98c08bf2e20d787be9b14c2cd94860 from master.