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

[Brian Bouterse]

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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