#20776: Multi-level Multi-table Inheritance - mismatched PKs / explicit
db_columns
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Reporter: dowstreet@… | Owner: nobody
Type: Bug | Status: new
Component: Uncategorized | Version: 1.5
Severity: Normal | Keywords: Muti-table Inheritance
Triage Stage: Unreviewed | Has patch: 0
Easy pickings: 0 | UI/UX: 0
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There seems to be a problem with queries that (i) span several levels of
multi-table inheritance subclassing when (ii) PKs do not match all the way
up the hierarchy. This situation can arise when a higher level object
(e.g. parent) is created before a lower level object (e.g. grandchild).
The query seems to use the PK of the grandchild's parent instead of
walking the whole way up the hierarchy when looking up fields that are
stored in the grandparent table.
In the example below there are three levels in the class hierarchy. When
accessing the name field (which is stored in the Level1 base class) from
an object in Level3, the query returns an unexpected value 'Top 1' instead
of the expected 'Bot 1':
{{{
# models.py
from django.db import models
class Level1(models.Model):
""" Top level base class """
level1_id = models.AutoField(primary_key=True, db_column='Level1_ID')
name = models.CharField(max_length=32, db_column='Name', blank=True,
null=True)
class Meta:
db_table = 'Level1'
class Level2(Level1):
""" Middle level class """
level2_id = models.AutoField(primary_key=True, db_column='Level2_ID')
level1 = models.OneToOneField('Level1', db_column='Level1_ID',
parent_link=True) # parent class
class Meta:
db_table = 'Level2'
class Level3(Level2):
""" Bottom level class """
level3_id = models.AutoField(primary_key=True, db_column='Level3_ID')
level2 = models.OneToOneField('Level2', db_column='Level2_ID',
parent_link=True) # parent class
class Meta:
db_table = 'Level3'
}}}
Using the shell to add a Level1 object and then a Level3 object to an
otherwise empty database:
{{{
from sc_test.models import *
>>> top1 = Level1()
>>> top1.name = 'Top 1'
>>> top1.save()
>>> bot1 = Level3()
>>> bot1.name = 'Bot 1'
>>> bot1.save()
>>> l1 = Level1.objects.all()
>>> l1
[<Level1: Level1 object>, <Level1: Level1 object>]
>>> l2 = Level2.objects.all()
>>> l2
[<Level2: Level2 object>]
>>> l3 = Level3.objects.all()
>>> l3
[<Level3: Level3 object>]
>>> l1[0].name
u'Top 1'
>>> l1[1].name
u'Bot 1'
>>> l2[0].name
u'Bot 1'
>>> l3[0].name
u'Top 1' # WRONG!! - expected value is u'Bot
1'
}}}
Note: In this example OneToOneField and ID/primary keys are manually added
so that specific db_column names can be used. It is unclear whether the
problem is related to this combination, or whether it affects auto-
generated OneToOneFields as well.
--
Ticket URL: <https://code.djangoproject.com/ticket/20776>
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