Hi, Check out https://code.djangoproject.com/ticket/9475
Seems like it would be fine if Django allowed add() and let any errors about missing data bubble-up. I personally think passing in a defaults dict (just like get_or_create does) would also be fine, but a callback seems like overkill. Here's a proof of concept: https://github.com/django/django/compare/master...collinanderson:ticket9475 Collin On Mon, Mar 20, 2017 at 5:46 PM, Luis Masuelli <luisfmasue...@gmail.com> wrote: > I was reading this link in the official history > <https://code.djangoproject.com/ticket/6095#comment:31> and this other > link in this group > <https://groups.google.com/forum/#!topic/django-developers/J-ycVFfgb4E>, > but still do not understand why the issue against a way to call .add() to > add a through model. I thought several approaches (they are all backward > compatible for the end-user) that could work: > > 1. Avoid the restriction to call add() if the model has only the two > FK fields. > 2. An additional way to call .add() specifying additional fields to > fill (this one is not mine; was discussed in the linked thread). You risk > getting a (wrapped?) exception if you do not populate other fields > appropriately. > 3. (This one was the first I thought and perhaps the easiest to > implement) Allow the ManyToManyField to specify a kwarg like > `through_factory=` which expects a callable which would populate more data. > The restriction to call .add() would remain if no `through_factory=` is > specified. > 4. Avoid the restriction to call delete() if the model has only the > two FK fields. > > I considered these cases: > > - It is quite trivial a model with only two fields, but perhaps the > intermediate models could have additional useful methods. I see no trouble > having such model and allowing .add() or .delete() calls there. > - Having a special factory method would allow calls to .add() since > we'd be providing a way to make .add() actually know how to create an > instance of the intermediate model. > - You can combine these points, implement one, none, or all of them. > - (I did not consider extended use cases for delete() intentionally, > since perhaps a strange model (with different field pairs) could fit in > different relationships, although I cannot think in a case with multiple > relationships not incurrin in, perhaps, duplicate data when tried to be > used as through= model, but anyway I prefer to keep silence for other cases > involving delete(), but the case I stated). > - It is up to the user to be careful regarding migrating an > intermediate model regarding adding, changing, or deleting fields. But > anyway, this applies to any modification in the database models right now. > - Points 1, 2, 3 and/or 4 could be implemented with no clash. A > combined approach of 1, 2, 3 would look like this (this flow only applies > for the case when the through= is present - such scenario right now only > consists of raising an exception; the case with no through= model would not > be affected at all): > - Instantiate `instance = ThroughModel(fka=a, fkb=b, > **kwargs_from_add)` with the respective model instances a and b, from > classes A and B which hold the desired M2M relationship. In this case, > the > point 2 just adds the **kwargs_from_add. If point 2 is not implemented, > no > **kwargs_from_add would be present. > - (*If point 3 is implemented*) Call the callable in > `through_factory=` invoking it `like_this(instance)`, if the callable is > present. It is expected to save the instance. > - (*If either point 1 is implemented and the model has only two > fields, or point 2 is implemented*) Manually save the instance (if > point 3 was not implemented or it was but the factory callable was not > specified). (*Otherwise - point 2 not implemented AND (point 1 not > implemented or model with more than two fields*)) Raise the > currently implemented exception for the .add() method with through= > specified (with a different string message) because the through_factory > was > not present, and so the framework does not know how to populate > additional > fields. > - Catch-and-reraise (or don't catch at all and let them be) the > error for missing data in the tried-to-save model. > - An example of the callable in point 3 would be like this (just an > example for, say, a game!): > - def on_added(through_model_instance): > through_model_instance.balance = 1000.0 #although this one > could be a default value at db level. > through_model_instance.save() > through_model_instance.achievements.create(tag='joined-this- > relation') > > I understand these approaches, combined or not, could not be perfect or > bug-free. But I'd like to discuss them instead of plainly discard them. > AFAIK right now the calls to .add() are disallowed when through is > specified. > > Could we work or at least discuss these use cases now? The history I > quoted seems to be pretty.... historic, and we're in 2017. The first thing > I'd like to know right not (with django 1.10 alive) is the feasability of > this feature in a future django version. > > I await your comments and/or criticism ^^. > > -- > You received this message because you are subscribed to the Google Groups > "Django developers (Contributions to Django itself)" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to django-developers+unsubscr...@googlegroups.com. > To post to this group, send email to django-developers@googlegroups.com. > Visit this group at https://groups.google.com/group/django-developers. > To view this discussion on the web visit https://groups.google.com/d/ > msgid/django-developers/ec5ffdfc-ef4b-4c30-a53d- > d427cbe053e2%40googlegroups.com > <https://groups.google.com/d/msgid/django-developers/ec5ffdfc-ef4b-4c30-a53d-d427cbe053e2%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 developers (Contributions to Django itself)" group. 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