Get list of values from grandchildren records-CodePudding

I'm trying to access the grandchildren records in a list to avoid duplicate records. In this example, a tag can only be used once across articles for a given author. I will use the resulting list of grandchildren records in my clean function to return validation errors.

class Article(models.Model):
    tag = models.ManyToManyField(Tag)
    author = models.ForeignKey(Author, on_delete=models.CASCADE)

class Tag(models.Model):


class Author(models.Model):

Right now I can do this:

print(author.articles.first().tag.first()) 
Travel

I'd like to be able to use something like author.articles.tags.all() to return the list and check the submitted form against it to raise a ValidationError message to the user.

How can this be done efficiently with the basic Many-to-Many setup without creating an intermediate table for the tag relationships? This is solely in the Admin interface, in case that matters at all.

CodePudding user response：

i come from the link you posted on upwork,

the way i understand your question,

what you want to achieve seems to be impossible ,

what i think can work , is to fetch articles related to the author, with their corresponding tags,

after that they are retrieved you do filtering and remove duplicates.

otherwise the tag has nothing to connect it with the author,,

CodePudding user response：

If my code looks like this

class Author(models.Model):
    name = models.CharField(max_length=255)
    created_at = models.DateTimeField(auto_now_add=True)
    updated_at = models.DateTimeField(auto_now=True)

class Tag(models.Model):
    tag_val = models.CharField(max_length=255)
    created_at = models.DateTimeField(auto_now_add=True)
    updated_at = models.DateTimeField(auto_now=True)

class Article(models.Model):
    author = models.ForeignKey(Author,related_name='articles', on_delete=models.CASCADE)
    tags = models.ManyToManyField(Tag, related_name='articles')
    created_at = models.DateTimeField(auto_now_add=True)
    updated_at = models.DateTimeField(auto_now=True)

I can write my query like this, assuming the variable 'author' has been assigned an Author object instance, and get non-duplicate Tag object instances for a users' articles

author.articles.values('tags').distinct()