Carl: Actually, the "subclass the operation" method fails for obvious reasons here:
https://github.com/django/django/blob/df0523debcc2d0984f1bc11d323f04227d4b388b/django/apps/registry.py#L202 I'm increasingly thinking that using MIGRATION_MODULES and just maintaining a local copy of Mezzanine's migration is the simplest solution. I sense that I'm going to face some resistance if I suggest changing their docs to reflect this. I'll start with a blog post. On 11/04/2014 04:30 PM, Justin Myles Holmes wrote: > Well, as I say, I don't particularly like this field injection notion, > and I'm happy to individually subclass where needed in my own projects. > > I'm less surefooted about issuing a pull request to mezzanine to alter > their docs to suggest doing this, although the doc is currently > ambiguous enough to be of little use anyway. > > I think that you and Carl have changed my mind about the > reasonableness of including migrations in one app for a model in > another. I'm no longer able to think of a scenario that justifies it. > > > On 11/04/2014 04:12 PM, Andrew Godwin wrote: >> Carl is right that this isn't a supported thing; Django models are >> designed to be static, and not mutate based on settings. >> >> Migrations are designed to only support one linear change history of >> models; swappable models are just about supported but not perfect and >> leads to issues. Adding in the ability to change or remove other >> fields based on installed models is kind of against the way >> migrations is designed and you'll have to do a lot of work to get >> around it. >> >> In this particular case, it's possible to make it work, but you'll >> have to subclass every one of the actions you want to use and provide >> a way to override the app_label it operates on, and manually >> construct the migrations to get the dependencies correct and >> non-circular (as I said, you'll have to do a lot of work to get >> around it). As long as you have anything resembling magically >> changing models in your project, you're not going to be able to use >> anything makemigrations outputs and expect it to Just Workâ˘. >> >> Andrew >> >> On Tuesday, November 4, 2014 3:59:23 PM UTC-8, Carl Meyer wrote: >> >> Hi Justin, >> >> On 11/04/2014 04:42 PM, Justin Myles Holmes wrote: >> >> I don't understand this use case. A ForeignKey impacts only >> one database >> > table's schema; the table for the model on which the ForeignKey >> field is >> > located. The schema of the model the ForeignKey points to >> doesn't need >> > to change at all based on the presence or absence of some other >> > ForeignKey pointing to it. >> > >> > Example: >> > >> > Two apps: apps.coconuts and apps.swallows, each with a >> canonical model >> > (Coconut and Swallow, respectively). >> > >> > The organization runs two version of the project: one with >> apps.swallows >> > and one without. >> > >> > The one with apps.swallows needs a field on Coconut, "carried," >> which is >> > a FK -=> Swallow. >> >> This is basically the same use case as the Mezzanine one - it's >> attempting to make a model more "reusable" by monkeypatching it with >> additional fields from outside. I think this should be avoided and >> discouraged, not better supported. >> >> The specifics of how to avoid would depend on details of the >> case, but I >> would look towards providing the reusable portions of the model >> as an >> abstract base, and maybe additionally using swappable models if >> other >> models need to have an FK to the "extendable" model. >> >> (Swappable models aren't yet formally supported for use outside >> contrib.auth, and there are still rough edges in migration's >> support for >> them, but I think moving them towards formal support is a much >> better >> answer to this general use case than monkeypatching fields onto >> models.) >> >> Another possibility (which I still don't like, but is better than >> monkeypatching) might be to conditionally define the Coconut.carried >> field, depending on the value of a setting. In that case, the right >> solution for migrations is to still define the migration that >> adds the >> Coconut.carried field in the coconuts app, but make it >> conditionally a >> no-op migration based on the value of the same setting. >> >> > To me, the proper way to do this is to ship a migration in the >> swallows >> > app which adds this field to apps.coconuts.models.Coconut. >> >> Sorry, I don't agree that there's anything "proper" about that >> :-) The >> migration is a secondary issue here - the basic problem is that >> you're >> monkeypatching the Coconut model, and I don't think migrations >> should >> support/encourage one app monkeypatching another app's models. >> >> > This is certainly a rare use case, but not a special case. On >> the other >> > hand, I don't see the advantage of restricting the models which >> may be >> > migrated to those in the app in which the migration appears, >> > particularly now that migration dependencies are more >> straightforward. >> >> It's a straightforward issue of encapsulation. The database >> schema for >> the models in app X is the responsibility of app X, not app Y. So >> far >> the only reasons I've seen why app Y might want to run a >> migration on an >> app-X model is because app Y is monkeypatching that model, to >> which I >> think the correct response is "don't do that," not "Django should >> better >> support that." >> >> >> I also wonder if there might be a way to do (2) without the >> > monkeypatching. That is, could Mezzanine provide an importable >> Operation >> > subclass for this purpose and document how to import and use it >> in a >> > migration? >> > >> > Well, the problem there is that Operation isn't used directly, but >> > subclassed for each individual type of migration operation. >> >> But for the specific Mezzanine use case, there are really only a few >> relevant migration operations, right? Seems like add-column, >> alter-column, and drop-column would cover it. >> >> Carl >> >> -- >> You received this message because you are subscribed to the Google >> Groups "Django users" group. >> To unsubscribe from this group and stop receiving emails from it, >> send an email to django-users+unsubscr...@googlegroups.com >> <mailto:django-users+unsubscr...@googlegroups.com>. >> To post to this group, send email to django-users@googlegroups.com >> <mailto:django-users@googlegroups.com>. >> Visit this group at http://groups.google.com/group/django-users. >> To view this discussion on the web visit >> https://groups.google.com/d/msgid/django-users/da3398bc-d175-432e-86f6-bbefad4ce9f8%40googlegroups.com >> <https://groups.google.com/d/msgid/django-users/da3398bc-d175-432e-86f6-bbefad4ce9f8%40googlegroups.com?utm_medium=email&utm_source=footer>. >> For more options, visit https://groups.google.com/d/optout. > -- You received this message because you are subscribed to the Google Groups "Django users" group. 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