Django Multiple join operations

Question

I have the following model with tables related through a Many-to-One Relationship.

class Reading(models.Model):
    reading = models.AutoField(primary_key=True)
    client_connection = models.ForeignKey(ClientConnection, models.DO_NOTHING, db_column='client_connection', blank=True, null=True)
    consumption = models.ForeignKey(Consumption, models.DO_NOTHING, db_column='consumption', blank=True, null=True)
    date = models.DateField(blank=True, null=True)

class Client(models.Model):
    client = models.AutoField(primary_key=True)
    zone = models.ForeignKey('Zone', models.DO_NOTHING, db_column='zone', blank=True, null=True)
    code = models.CharField(unique=True, max_length=255, blank=True, null=True)
    order = models.IntegerField(blank=True, null=True)
    full_name = models.CharField(max_length=255, blank=True, null=True)

class Zone(models.Model):
    zone = models.AutoField(primary_key=True)
    description = models.CharField(max_length=255, blank=True, null=True)
    code = models.CharField(unique=True, max_length=255, blank=True, null=True)

Using Django ORM's select_related() I can create a QuerySet that will follow the foreign-key relationship selecting additional related-objects as follows.

def client_meters(request):
    query = Reading.objects.filter(date__gte=datetime.date(2018,10,31)).select_related('client_connection__client__zone').all()
    c_c = query.client_connection

However, I get the error

c_c = query.client_connection
AttributeError: 'QuerySet' object has no attribute 'client_connection'

What am I missing.

This doesn't have anything to do with joins.

Daniel Roseman
– Daniel Roseman

2018-12-19 16:59:11 +00:00
Commented Dec 19, 2018 at 16:59 — Daniel Roseman
– Daniel Roseman, Commented Dec 19, 2018 at 16:59

vishes_shell · Accepted Answer · 2018-12-19 16:35:44Z

3

That's because you are trying to access client_connection attribute of model instance on model queryset.

This will do:

for obj in query:
    c_c = obj.client_connection
    # do what you want

In simple terms: you are trying to access attribute of list of elements instead of single element.

answered Dec 19, 2018 at 16:35

vishes_shell

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