Can Django queryset generate a SQL statement with self join? - django

Question

I have a table that relates parents to children, it has the following data:
+-----+-----+-----+--------+
| pid | rel | cid | relcat |
+-----+-----+-----+--------+
| 13 | F | 216 | 1 |
| 13 | F | 229 | 1 |
| 13 | f | 328 | 2 |
| 13 | F | 508 | 1 |
| 13 | F | 599 | 1 |
| 13 | f | 702 | 2 |
| 560 | M | 229 | 1 |
| 560 | m | 702 | 2 |
+-----+-----+-----+--------+
I can find brothers of 229 by joining npr table to itself with SQL:
SELECT npr_a.cid,
CASE (SUM(IF(npr_a.relcat=1 AND npr_b.relcat=1,1,0))) WHEN 2 THEN '~FB~' WHEN 1 THEN '~HB~' ELSE '~Foster~' END AS BrotherType,
abs(person_details.isalive) as isalive
FROM person_details,
npr npr_a,
npr npr_b
WHERE ( npr_b.cid = 229) AND
( npr_a.pid = npr_b.pid ) AND
( npr_a.cid <> 229) AND
( npr_b.relcat <> 3 ) AND
( npr_a.relcat <> 3 ) AND
( person_details.id = npr_a.cid )
GROUP BY npr_a.cid;
to get:
+-----+-------------+---------+
| cid | BrotherType | isalive |
+-----+-------------+---------+
| 216 | ~HB~ | 1 |
| 328 | ~Foster~ | 0 |
| 508 | ~HB~ | 0 |
| 599 | ~HB~ | 0 |
| 702 | ~Foster~ | 1 |
+-----+-------------+---------+
I tried many ways to get it using Django queryset but all failed to get the correct results. the best thing I was able to get is:
idp = Npr.objects.filter(cid=229).values_list('pid', flat=True)
idc = Npr.objects.filter(pid__in=idp).exclude(cid=229)
but that solution was not able to generate the BrotherType field.
My models:
class PersonDetails(models.Model):
id = models.AutoField(db_column='ID', primary_key=True)
name= models.CharField(db_column='Name', max_length=20, blank=True, null=True)
isalive = models.BooleanField(db_column='isAlive')
class Meta:
managed = False
db_table = 'person_details'
class Npr(models.Model):
rid = models.AutoField(db_column='rid', primary_key=True)
pid = models.ForeignKey(PersonDetails, on_delete=models.CASCADE, db_column='PID')
rel = models.CharField(max_length=1)
cid = models.ForeignKey(PersonDetails, on_delete=models.CASCADE, db_column='CID')
relcat = models.PositiveIntegerField()
class Meta:
managed = False
db_table = 'npr'
unique_together = (('pid', 'cid'),)
using Django Version: 3.0.4 Python version: 3.7.3 Database: 10.3.22-MariaDB-0+deb10u1-log
Any suggestion to build the required queryset?

Answer 1

luckily I found a solution to my problem.
I created a new model pointing to the same database table
class nNpr(models.Model):
rid = models.AutoField(db_column='rid', primary_key=True)
pid = models.ForeignKey(Npr, on_delete=models.CASCADE, db_column='PID', to_field='pid', related_name='rel_pid')
rel = models.CharField(max_length=1)
cid = models.PositiveSmallIntegerField(db_column='CID')
relcat = models.PositiveIntegerField()
class Meta:
managed = False
db_table = 'npr'
unique_together = (('pid', 'cid'),)
and modified the original model to suppress warning that Django generates when you join a foreign key field to a non unique field, the modified model is:
class Npr(models.Model):
rid = models.AutoField(db_column='rid', primary_key=True)
pid = models.OneToOneField(PersonDetails, on_delete=models.CASCADE, , unique=True, db_column='PID', related_name='rel_pid')
rel = models.CharField(max_length=1)
cid = models.ForeignKey(PersonDetails, on_delete=models.CASCADE, db_column='CID', related_name='rel_cid')
relcat = models.PositiveIntegerField()
class Meta:
managed = False
db_table = 'npr'
unique_together = (('pid', 'cid'),)
the last thing is the query set:
brothers = nNpr.objects.select_related('pid').filter(cid = 229).exclude(Q(relcat = 3)\
| Q(pid__cid = 229) | Q(pid__relcat = 3 )).values('pid__cid', 'pid__cid__name',\
'pid__cid__isalive').annotate(cprel=Sum('relcat'), pcrel=Sum('pid__relcat'))
and the resulting SQL is:
SELECT T2.`CID`, `person_details`.`Name`, `person_details`.`isAlive`, SUM(`npr`.`relcat`) AS `cprel`\
, SUM(T2.`relcat`) AS `pcrel` FROM `npr` INNER JOIN `npr` T2 ON (`npr`.`PID` = T2.`PID`) INNER\
JOIN `person_details` ON (T2.`CID` = `person_details`.`ID`) WHERE (`npr`.`CID` = 229 AND NOT \
((`npr`.`relcat` = 3 OR T2.`CID` = 198 OR T2.`relcat` = 229))) GROUP BY T2.`CID`,\
`person_details`.`Name`, `person_details`.`isAlive` ORDER BY NULL

Answer 2

I found another better solution that doesn't need to create a new model pointing to the same database table. The solution was cascading the required relations recursively on one model:
brothers = Npr.objects.filter(pid__pid__cid = 229, relcat__lt = 3,
pid__pid__relcat__lt = 3).values('cid', 'cid__name')