[Django] #33313: Inheriting from multiple abstract models with same field causes name collision when overriding field is direct parent

Django

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Nov 23, 2021, 3:03:09 PM11/23/21

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#33313: Inheriting from multiple abstract models with same field causes name
collision when overriding field is direct parent
-------------------------------------+-------------------------------------
Reporter: Ben Nace | Owner: nobody
Type: Bug | Status: new
Component: Database | Version: 3.2
layer (models, ORM) |
Severity: Normal | Keywords:
Triage Stage: | Has patch: 0
Unreviewed |
Needs documentation: 0 | Needs tests: 0
Patch needs improvement: 0 | Easy pickings: 0
UI/UX: 0 |
-------------------------------------+-------------------------------------
Given the following example models:

{{{
class ModelActivation(models.Model):
start_date = models.DateField(null=True, blank=True)
end_date = models.DateField(null=True, blank=True)
active = models.BooleanField()

class Meta:
abstract = True

class BaseData(ModelActivation):
entity_state = models.CharField(max_length=100)

class Meta:
abstract = True

class RequiredStart(models.Model):
start_date = models.DateField()

class Meta:
abstract = True

class RequiredEnd(models.Model):
end_date = models.DateField()

class Meta:
abstract = True

class RequiredStartEnd(RequiredStart, RequiredEnd):
class Meta:
abstract = True
}}}
\\

Any of the following when the override for start_date is defined on a
direct parent model, results in the error "(models.E006) The field
'start_date' clashes with the field 'start_date' from model
'app.testmodel' (or 'app.testmodel2')"
{{{
class TestModel(RequiredStart, BaseData):
pass

class TestModel2(RequiredStart, ModelActivation):
pass
}}}
However, if the overriding field is pushed up to a grandparent model,
rather than a direct parent, it works fine.
{{{
class TestModel3(RequiredStartEnd, BaseData):
pass

class TestModel4(RequiredStartEnd, ModelActivation):
pass
}}}

In my limited debugging, it appears to me that this is because of the way
inherited_attributes is tracked in the __new__ method of the ModelBase
model metaclass (in django.db.models.base.py). For a grandparent model,
not being a direct parent, all items in the __dict__ will be added to
inherited_attributes, which includes the fields:

{{{
if base not in parents or not hasattr(base, '_meta'):
# Things without _meta aren't functional models, so
they're
# uninteresting parents.
inherited_attributes.update(base.__dict__)
continue
}}}

However, when a field is inherited from a direct parent, it is not added
to inherited_attributes, it is not added to field_names, and it does not
appear in new_class.__dict__, so the field from the ancestor higher up in
the mro is also added via

{{{
for field in parent_fields:
if (field.name not in field_names and
field.name not in new_class.__dict__ and
field.name not in inherited_attributes):
new_field = copy.deepcopy(field)
new_class.add_to_class(field.name, new_field)
}}}

--
Ticket URL: <https://code.djangoproject.com/ticket/33313>
Django <https://code.djangoproject.com/>
The Web framework for perfectionists with deadlines.

Django

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Nov 24, 2021, 2:47:14 PM11/24/21

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#33313: Inheriting from multiple abstract models with same field causes name
collision when overriding field is direct parent
-------------------------------------+-------------------------------------
Reporter: Ben Nace | Owner: nobody
Type: Bug | Status: new

Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
-------------------------------------+-------------------------------------

Changes (by Mariusz Felisiak):

* cc: Jarek Glowacki, Carlton Gibson (added)

--
Ticket URL: <https://code.djangoproject.com/ticket/33313#comment:1>

Django

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Nov 24, 2021, 3:00:11 PM11/24/21

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#33313: Inheriting from multiple abstract models with same field causes name
collision when overriding field is direct parent
-------------------------------------+-------------------------------------
Reporter: Ben Nace | Owner: nobody
Type: Bug | Status: new

Has patch: 0 | Needs documentation: 0

Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
-------------------------------------+-------------------------------------

Comment (by Ken Whitesell):

Ben, this appears to be fixed in the Django 4.0 pre-release version.

I can recreate it using your example under 3.2.9, but not under 4.0.pre.

--
Ticket URL: <https://code.djangoproject.com/ticket/33313#comment:2>

Django

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Nov 24, 2021, 3:12:59 PM11/24/21

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#33313: Inheriting from multiple abstract models with same field causes name
collision when overriding field is direct parent
-------------------------------------+-------------------------------------
Reporter: Ben Nace | Owner: nobody

Type: Bug | Status: closed

Component: Database layer | Version: 3.2
(models, ORM) |

Severity: Normal | Resolution: fixed
Keywords: | Triage Stage:
| Unreviewed
Has patch: 0 | Needs documentation: 0

Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
-------------------------------------+-------------------------------------