[Pulp-dev] Master-detail inheritance in Pulp 3

[David Davis]

The default_related_name setting is something that django provides.
Subclasses can also explicitly define their OneToOneField parent link as
well:

content_ptr = models.OneToOneField(Content, on_delete=models.CASCADE,
parent_link=True, related_name='rpm_package')

I am not sure what you mean by 'robust' but if a plugin subclass doesn't do
either of these, it may not work with other plugins.

I think the question now would be whether we should just document this or
try to do it automagically for plugins?

David


On Tue, Apr 23, 2019 at 12:31 PM Brian Bouterse <bbouters at redhat.com> wrote:

>
>
> On Tue, Apr 23, 2019 at 11:02 AM David Davis <daviddavis at redhat.com>
> wrote:
>
>> I think I found another solution that might work best: defining
>> 'default_related_name' on subclassed master-detail models. So Package in
>> pulp_rpm would define its default_related_name as "rpm_package".
>>
> Would we be making 'default_related_name' or is that something Django is
> providing? If it's something Pulp would be providing perhaps defining the
> explicit one-to-one field is better. Any plugin that takes the step of
> defining the one-to-one field will insulate themselves from other plugins.
> If plugins don't take that step they will still work, just not as robustly.
> Am I thinking about this correctly?
>
>
>> David
>>
>>
>> On Tue, Apr 23, 2019 at 10:29 AM David Davis <daviddavis at redhat.com>
>> wrote:
>>
>>> I wanted to email the pulp-dev list about a major problem[0] that was
>>> recently encountered in Pulp 3 that affects how the Pulp 3 plugin API
>>> functions.
>>>
>>> # Problem
>>>
>>> In the plugin API we rely on inheritance to allow plugin writers to
>>> import functionality into their plugin. This includes models such as Remote
>>> and Content that are inherited by plugins. We rely on django's multi-table
>>> inheritance[1] for these models.
>>>
>>> Behind the scenes, django defines a OneToOneField and a reverse
>>> accessor. This field is not namespace so if two subclasses have the same
>>> name, you get an error ("Reverse accessor for 'Package.content_ptr' clashes
>>> with reverse accessor for 'Package.content_ptr'.")
>>>
>>> To give an actual example, both the Debian and RPM plugins implement a
>>> Package class. This causes an error to be raised when a user installs both
>>> plugins. Django tries to define a 'package' reverse accessor for both
>>> subclasses and blows up.
>>>
>>> # Potential Solutions
>>>
>>> ## Class Naming
>>>
>>> The first solution I can think of which is probably also the simplest
>>> and most straightforward would be to require plugin writers to namespace
>>> their master/detail subclass names. So Package would be RpmPackage. This
>>> places the onus on plugin writers to name their master/detail classes
>>> correctly.
>>>
>>> ## Defining OneToOneField
>>>
>>> The other solution would be to either manually define the OneToOneField
>>> on the subclasses[2] and specify a namespaced field name. There may be a
>>> way to do this dynamically (ie magically) in the parent somehow as well.
>>>
>>> ## Abstract Class
>>>
>>> Lastly, we could redefine master models as abstract classes[3]. I can
>>> think of at least one or two places (e.g. content field on
>>> RepositoryVersionContent, publisher field on Publication) that would have
>>> to switch their relationships to generic relationships in order to
>>> accommodate this change.
>>>
>>> There might be other solutions I am not thinking of so feel free to
>>> propose something. Also, quick feedback would be greatly appreciated as
>>> this is going to be a major change in our plugin API.
>>>
>>> [0] https://pulp.plan.io/issues/4681
>>> [1]
>>> https://docs.djangoproject.com/en/2.2/topics/db/models/#multi-table-inheritance
>>> [2]
>>> https://docs.djangoproject.com/en/2.2/topics/db/models/#specifying-the-parent-link-field
>>> [3]
>>> https://docs.djangoproject.com/en/2.2/topics/db/models/#abstract-base-classes
>>>
>>> David
>>>
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>
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