How to improve str() representation of relationships? Eg: when deleting from admin

[nemesifier]

I have two models: Config and Template (django-netjsonconfig), Config has a sortedm2m relation to Template.

When deleting an object which has m2m relationships from admin interface, django says:

Are you sure?

    Are you sure you want to delete the selected template? All of the following objects and their related items will be deleted:

Summary

    - Config-template relationships: 6
    - Templates: 1

Objects

    - Template: [Generic-OpenWRT] Wireless settings
        - Config-template relationship: Config_templates object
        - Config-template relationship: Config_templates object
        - Config-template relationship: Config_templates object
        - Config-template relationship: Config_templates object
        - Config-template relationship: Config_templates object
        - Config-template relationship: Config_templates object

The repetition of Config-template relationship: Config_templates object conveys no information. I would like to display something more useful.

For example, each config has a name, let's say I'm deleting a template which was installed on two config objects named Auditorium and Gym

I've been able to accomplish this only through monkey patching:

class Config(models.Model):
    # ...
    pass

def sortedm2m__str__(self):
    """
    Improves string representation of m2m relationship objects
    """
    return self.config.name

Config.templates.through.__str__ = sortedm2m__str__

But I'm wondering if there's a better way. Any suggestion?

[gregmuellegger]

No I have no good solution to this with the current implementation of sortedm2m. But we could ofcourse extend the SortedManyToManyField to support this usecase. I see the need you have and saw those not-so-nice string representations also a couple of times already.

1. Idea: Add a to_string argument to the field so you can provide a function that gets a, b and sort_value as arguments, where as a is an instance of the from-model, Config in your case, and b is an instance of the to-model, so Template in your case.

2. Idea: Provide a base_class argument to the field, so you can modify the base class that the through-model inherits from. This would allow you to customize the __str__ method by defining it in an abstract base class.

What do you think? Would you be interested in working on a solution to your problem. I'm happy for any pull request.

[nemesifier]

Both ideas would work. I'm inclined to prefer 2 because I think that would allow to customize more logic of the automatically generated "through" model. Is my supposition right? Yes I can work on this, I don't like to leave that monkey-patching in django-netjsonconfig.

[nemesifier]

@gregmuellegger have you had any time to think about it? Would you prefer idea1 or idea2?

[nemesifier]

@gregmuellegger I'm still keen to work on this.

[gregmuellegger]

Yes, I would go with your recommendation. The base_class seems super flexible and good for a lot of other customizations. So option 2 has my vote as well.

Thanks for your on going interest :)

[rohithasrk]

Working on this.

[rohithasrk]

By reading the above thread and also the code, I've understood the following.

• We need to customise the __str__ method of the through model to print something that would be passed as an argument.

I've got some queries regarding this,

• There needs to be a base_class defined in SortedManyToManyField class, isn't it?
• What exactly needs to be inside the __str__ method of through model? How do we achieve that level of abstraction?
• "This would allow you to customize the str method by defining it in an abstract base class." - Can you please elaborate?

@nemesisdesign @gregmuellegger Please help.

[nemesifier]

@rohithasrk:

yes we need to add a base_class parameter to SortedManyToManyField ;

by default the __str__ method does not have to do anything in particular, it will be the responsibility of the users of django-sortedm2m to supply a specialized __str__ method; it may as well be absent;

we need to add a way to supply a base class for the through model of SortedManyToManyField in order to customise it, at the moment it is not possible to do this without resorting to monkey patching.

[rohithasrk]

So, I understand that I need to do two things,

• Add a base_class parameter to SortedManyToManyField which would be by defining a function to add the field. (Different from normally adding the field to a model) -- Still confused in this though.
• Add a function named modify_through (for ex.) which modifies the through model of SortedManyToManyField. And modify_through shall look somewhat like this,

def modify_through(self, base_class, some_function):
    self.through.__str__ = base_class.some_function

Is it right? @nemesisdesign

[nemesifier]

not right, you have to find a way to pass a base class that django-sortedm2m can use as a base for its through model. Look for create_intermediate_model and similar function in the codebase. You will have to read the internal code of this project.

[rohithasrk]

I see. I'll go through once.

[rohithasrk]

@nemesisdesign @gregmuellegger : Please review PR #101