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

David Davis daviddavis at redhat.com
Mon Apr 29 18:16:27 UTC 2019

It seems like most people are in favor of setting the OneToOneField or
perhaps the default_related_name on the detail model. I think there’s also
some interest in seeing how we can do this automatically for plugins. I’ve
added this feedback to the issue:



On Wed, Apr 24, 2019 at 6:22 AM Ina Panova <ipanova at redhat.com> wrote:

> I would avoid making changes in class naming. So +1 for the OneToOneField
> definition.
> --------
> Regards,
> Ina Panova
> Software Engineer| Pulp| Red Hat Inc.
> "Do not go where the path may lead,
>  go instead where there is no path and leave a trail."
> On Tue, Apr 23, 2019 at 6:45 PM David Davis <daviddavis at redhat.com> wrote:
>> 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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>>>> Pulp-dev at redhat.com
>>>> https://www.redhat.com/mailman/listinfo/pulp-dev
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