tags.add function not working in pre-save/post-save in django

[naveedur]

I am using django taggit for tags. I want to add fields text in tags for some data table. But its not working.
models.py:

class resource(models.Model):

    title=models.CharField(max_length=100)
    size=models.CharField( max_length=20, default="")
    desc=models.TextField(default="")
    file=models.FileField(default="", blank=True)
    url= models.URLField(max_length=200, blank=True)
    varient=models.CharField(max_length=100, default="")
    Brand = models.ForeignKey(brand,on_delete=models.CASCADE, default="")
    Model = models.ForeignKey(model,on_delete=models.CASCADE, default="")
    Categories = models.ForeignKey(category,on_delete=models.CASCADE, default="")
    update_at=models.DateField(auto_now=True)
    slug=models.SlugField(default="", unique=True, blank=True)
    tags = TaggableManager(blank=True)

pre-save function:

def tag_set(sender, instance, **kwargs):

    ans=[instance.title ,instance.Brand,instance.Model]
    instance.tags.add(*ans)

pre_save.connect(tag_set, sender=resource)

It's neither showing any error nor working. Any one please help if any solution or alternative.

[rtpg]

Does this code not work if you use the post_save hook instead of the pre_save hook? pre_save will likely run into issues with the instance changing, but post_save should probably work.

[naveedur]

@rtpg No it's also not working for post-save

[rtpg]

Could you do something like:

print(instance.tags.all())
print(ans)
instance.tags.add(*ans)
print(instance.tags.all())

and show the results here? that might make it a bit easier to debug what is going on

[naveedur]

after that I got this ;

<QuerySet []> ['tgas-title', 'description', '4G'] <QuerySet [<Tag: description>, <Tag: 4G>, <Tag: tgas-title>]>

But tags are not saved in data table tags field.

[naveedur]

@rtpg please any suggestion for that

[chambersh1129]

I made some progress on tracking this down but not entirely sure how to address it. Looks like when creating/editing an object in the admin view with a post_save signal the order of operations is:

• model.save() function is run
• post_save signal is fired off, tags are updated
• TaggableManager.save_form_data() is run, which runs the _TaggableManager.set() function, replacing the post_save tags with what is in the text field when the admin view form was submitted.
• since model.save() isn't run again, the post_save doesn't run a second time

Looks like the tag is added with the post_save then overwritten with the .set() in the save_form_data. I can work on a PR to try to address this, thoughts on the best method?

[chambersh1129]

Made a little more progress identifying the issue:

• In Admin view, the form is saved with commit=False
• instead of running form._save_m2m(), commit=False sets form.save_m2m = form._save_m2m to be deferred and run later
• model.save() method gets called, followed by post_save signal which now adds the tag
• model instance at this point has original tags, any tag(s) added in post_save signal, but NO tags added/removed using the form
• ModelAdmin.save_related() is then called which finally runs form.save_m2m
• ModelForm._save_m2m() runs save_form_data()
• save_form_data() runs the TaggitManager.set(), overwriting the current tags with what was in the submitted form (pre post_save adding tag)
• since the model has already been saved, it isn't called again and therefore the post_save signal isn't run again

This is why printing the tags before and after the add in the post_save shows the correct tags, but once saved the post-save tags are gone. The solution to form.save(commit=False) is listed in the docs. I've played around with it a little, attempting to overwrite various save methods and setting custom forms for the Admin view but everything I've tried brings other problems.

I'm open to ideas. I'll keep investigating as I have time.

[rtpg]

@chambersh1129 thanks for digging into this, I get what you're saying.

I don't think this is really a bug, signals are really nastly like this. For me what I do in these situations is to just give in and write a update_default_tags function that I call manually in an admin form subclass or the like. Ultimately in a form save, if we get a list of tags, calling set instead of add is the least bugful thing to do here.

So yeah, recommendation here is to overwrite the Admin form (there's a save_related method) or otherwise do stuff to confirm that these tags are present/in-sync.

[rtpg]

I'm labelling this a bug but it's not really a bug IMO. Just thinking about what the right way forward can be for this kind of thing