如何根据具有相同字段值的对象总数在 model 中添加自动增量字段？

Question

I'm new to the whole Django thing and a bit lost. Sorry if the title is a bit confusing I'll try to clear things out.

So basically I have two models ( Folder and Document ). A Document can have a single Folder as one of its fields using a Foreign Key . Now I have another field in Document that needs to get the value of the total Document objects that share the same Folder and increase by one.

I've tried things I read on the docs ( aggregation, F() objects, overriding model's save() function ) as well as some answers is read here but didn't manage to get it right so I'm posting to get some help. Below is my models.py file with the two models and some comments for better understanding.

class Folder(models.Model):
    category = models.IntegerField()
    subCategory = models.IntegerField()
    name = models.CharField(max_length= 50)
    desc = models.TextField()

class Document(models.Model):
    folder = models.ForeignKey(Folder, on_delete=models.CASCADE)
    date_created = models.DateField()
    date_added = models.DateTimeField()

    #The field below needs to sum all Document objects that share 
    #the same folder value in the database + 1 and set it as its default value
    f_no = models.IntegerField(default=lambda: Document.objects.aggegate(Count('folder')) + 1)

Thank you in advance, any leads or clues are most welcome

EDIT: Forgot to say that all management is done via Django's admin dashboard if this has anything to do at all with my situation. I registered both models in admin.py and that's all. I make new Folder objects when needed and save Documents with one specific Folder in them each time

Answer 1

I would recommend creating a ManyToMany relation in the Folder, and add a created Document object into the Folder's ManyToMany relation.

Models.py

class Folder(models.Model):
    category = models.IntegerField()
    subCategory = models.IntegerField()
    name = models.CharField(max_length= 50)
    desc = models.TextField()
    documents = models.ManyToManyField('app.Document')

You can add can add documents to the folder by using .add() to the ManyToMany relation and the amount of documents in the relation by using .count()

ManyToMany relations are well documented here: https://docs.djangoproject.com/en/3.1/topics/db/examples/many_to_many/

Answer 2

When you create a Document model object, it represents a single item. That single item shouldn't have a count of how many documents are in a file. The f_no you note should actually be in the Folder model.

Once you create a Document object that is related to a Folder object via ForeignKey, you can use signals to increment the f_no field that resides in the Folder object.

@receiver(post_save, sender=Document)
def increment_folder_item_count(sender, **kwargs):
    # get Folder object via Document model instance folder foreignkey field
    # folder.f_no += 1
    # folder.save()