Model multiple inheritance primary key reuse

[Erik Wognsen]

This is easiest to explain with an example.

Using the following models the primary keys may be reused for unrelated objects in the Base table.

class Base(models.Model):
	def __unicode__(self):
		return '(id %d)' % (self.id)

class OtherBase(models.Model):
	def __unicode__(self):
		return '(id %d)' % (self.id)

class ChildOne(Base):
	pass

class ChildTwo(OtherBase, Base):
	pass

Here are sessions demonstrating the behaviors with three different parentages for ChildTwo. Each session starts with a fresh database.

# With ChildTwo(Base)
In [1]: from someapp.models import *
In [2]: ChildOne.objects.create()
Out[2]: <ChildOne: (id 1)>

In [3]: ChildTwo.objects.create()
Out[3]: <ChildTwo: (id 2)>

In [4]: Base.objects.all()
Out[4]: [<Base: (id 1)>, <Base: (id 2)>]

# As expected


# With ChildTwo(Base, OtherBase)
In [1]: from someapp.models import *
In [2]: ChildOne.objects.create()
Out[2]: <ChildOne: (id 1)>

In [3]: ChildTwo.objects.create()
Out[3]: <ChildTwo: (id 2)>

In [4]: Base.objects.all()
Out[4]: [<Base: (id 1)>, <Base: (id 2)>]

In [5]: OtherBase.objects.all()
Out[5]: [<OtherBase: (id 2)>]

# As expected


# With ChildTwo(OtherBase, Base)
In [1]: from someapp.models import *
In [2]: ChildOne.objects.create()
Out[2]: <ChildOne: (id 1)>

In [3]: ChildTwo.objects.create()
Out[3]: <ChildTwo: (id 1)>

In [4]: Base.objects.all()
Out[4]: [<Base: (id 1)>]

In [5]: OtherBase.objects.all()
Out[5]: [<OtherBase: (id 1)>]

# Not expected, the two objects share the id!


# With ChildTwo(OtherBase, Base), creating a ChildTwo instance first
In [1]: from someapp.models import *
In [2]: ChildTwo.objects.create()
Out[2]: <ChildTwo: (id 1)>

In [3]: ChildOne.objects.create()
Out[3]: <ChildOne: (id 2)>

In [4]: Base.objects.all()
Out[4]: [<Base: (id 1)>, <Base: (id 2)>]

In [5]: OtherBase.objects.all()
Out[5]: [<OtherBase: (id 1)>]

# As expected


# Continuing...
In [6]: ChildTwo.objects.create()
Out[6]: <ChildTwo: (id 2)>

# Again, an id reuse.

In general the ChildTwo creation just picks the next available id in OtherBase without checking if it's free in Base.

This was observed using Django 1.3.0 and behavior was the same for both sqlite3 (3.7.2) and mysql (5.1.49).

[Karen Tracey]

This looks like a dupe of #12002.