Howto do a LEFT JOIN in Django - django

Question

Hello there (or as we say Moin Moin)!
I am new to django development (version 2.0) and do not understand how to do a LEFT JOIN in django-syntax.
For example I have the following models:
class Units(models.Model):
UnitID = models.AutoField(primary_key=True)
Description = models.CharField(max_length=30)
class MappingOperatorUnits(models.Model):
OperatorID = models.ForeignKey(User, on_delete=models.CASCADE)
UnitID = models.ForeignKey('Units', on_delete=models.CASCADE)
class Participants(models.Model):
UnitID = models.ForeignKey('Units', on_delete=models.CASCADE)
LessonID = models.ForeignKey('Lessons', on_delete=models.CASCADE)
OperatorID = models.ForeignKey(User, on_delete=models.PROTECT)
NumberParticipants = models.SmallIntegerField()
Now I am trying to do a query like
SELECT *
FROM MappingOperatorUnits
LEFT JOIN Units
ON ON MappingOperatorUnits.UnitID = Units.UnitID
LEFT JOIN Participants
ON MappingOperatorUnits.UnitID = Participants.UnitID
and what I want to be the result is something like this:
+-------------------+-----------------------+---------------------------------+
| Units.Description | Participants.LessonID | Participants.NumberParticipants |
+-------------------+-----------------------+---------------------------------+
| TeamA | 1 | 0 |
+-------------------+-----------------------+---------------------------------+
| TeamB | 1 | 3 |
+-------------------+-----------------------+---------------------------------+
| TeamC | NULL | NULL |
+-------------------+-----------------------+---------------------------------+
| TeamA | 2 | 2 |
+-------------------+-----------------------+---------------------------------+
| TeamB | 2 | 5 |
+-------------------+-----------------------+---------------------------------+
| TeamC | 2 | 1 |
+-------------------+-----------------------+---------------------------------+
I tried a lot of things in manage.py's shell but din't come to the solution. Anybody can help me to get it? Thank you!

Answer 1

You can annotate use .values(…) [Django-doc] to load values of a referenced model:
MappingOperatorUnits.objects.values(
'UnitID__Description',
'UnitID__participants__LessonID',
'UnitID__participants__NumberParticipants'
)
This produces a query that looks like:
SELECT units.Description,
participants.LessonID_id
participants.NumberParticipants
FROM mappingoperatorunits
INNER JOIN units ON mappingoperatorunits.UnitID_id = units.UnitID
LEFT OUTER JOIN participants ON units.UnitID = participants.UnitID_id
The INNER JOIN is an optimization that can be done here, since it is a non-NULLable ForeignKey, and thus we know there is always a related Units record.
Note: normally a Django model is given a singular name, so Unit instead of Units.
Note: Normally one does not add a suffix _id to a ForeignKey field, since Django
will automatically add a "twin" field with an _id suffix. Therefore it should
be unit, instead of UnitID.