Django ORM查询从一个表中检索对象，并从另一个与外键相关的表中检索最新对象？

Question

如何将下面的SQL查询转换为Django ORM语句，以便获得相同的字段？

SELECT 
    entity_a.id AS entity_a__id,
    entity_a.created_at AS entity_a__created_at,
    MAX(entity_b.last_update) AS max__entity_b__last_update,
    entity_b.id AS entity__b_id,
    entity_b.datadump AS entity_b__datadump
FROM
    entity_a
        INNER JOIN
    entity_b ON entity_a.id = entity_b.entity_a_id
WHERE
    entity_a.created_at < '2020-03-10'
GROUP BY entity_a.id;

Answer 1

从django>=1.8开始，您可以将annotate和F对象用作字段别名。对于聚合，您可以使用aggregation features of the ORM

from django.db.models import F, Max

EntityA.objects.annotate(
    entity_a__id=F('id'),
    entity_a__created_at=F('created_at'),
    entity__b_id=F('entity_b_id'),
    entity_b__datadump=F('entity_b__datadump'),
    max__entity_b__last_update=Max('entity_b__last_update')
).filter(
    created_at__lt='2020-03-10',
).values(
    'entity_a__id',
    'entity_a__created_at',
    'entity__b_id',
    'entity_b__datadump',
    'max__entity_b__last_update'
)