Django：通过自动将计数器附加到title / slug来保存唯一条目

Question

I have a Django form entry where I want the model saved to the database to have a unique title, or rather a unique "slug" (derived from the title). I do this by appending a counter to the slug. So something like "this-is-a-title-1" and possibly then later "this-is-a-title-2".

But if two users at the same time submit an entry with the same title, only one entry will make it to the database (the slug field is unique); saving the other will throw an IntegrityError. To create a unique counter, I now loop upon encountering such aa duplicate entry, as follows:

class MyClass(models.Model):
    self.title = models.CharField()
    self.slug = models.SlugField(unique=True, blank=False)

    def save(*args, **kwargs):
        baseslug = slugify(self.title)
        self.slug = baseslug + "-1"
        count = 1
        while count < MAXCOUNT:  # MAXCOUNT something like 1000
            try:
                super(MyClass, self).save(*args, **kwargs)
            except IntegrityError as exc:
                count += 1
                self.slug = baseslug + "-{:d}".format(count)
            else:
                break  # we're fine; break out of the loop

I do this through the admin, where the slug is not entered explicitly (but is created in the save() method).

My question is : is this a good idea? Somehow it feels like a work-around that has a better solution.

Another solution I can think of is to pop-up a message to the unfortunate second user, but that would only say that "your entry can't be saved at this time. Try again in a few seconds." I would then also first need to get the current count from existing (identical-except-for-the-counter) slugs. The above method automatically avoids that, although it will run into problems if ever there are more than MAXCOUNT identical slugs.

Are there any other suggestions, or does this seems like a viable solution?

(Note: I came across this similar question suggested by SO while entering my question, but there, a get() is first performed before saving the entry. This potentially allows enough time between the get() and save() to still attempt to save duplicate entries, thus causing an IntegrityError. Otherwise it appears to suggest pretty much the same solution as I have now.)

Answer 1

If you're just needing to have a value to pass through the querystring, you could just add a property to the model, that combines the slug + the pk:

class MyClass(models.Model):
    title = models.CharField(max_length=255)
    slug = models.SlugField(max_length=300)

    @property
    def url_slug(self):
        return '{}-{}'.format(self.slug, self.id)

Then you don't have to worry about counts being off, race conditions, etc and you only have one write operation.

Answer 2

You could do two writes in your save method:

1. Save the instance with a temporary random slug (to satisfy uniqueness constraint)
2. Calculate the slug using the PK and save again

Something like this:

class MyClass(models.Model):
    title = models.CharField(max_length=255)
    slug = models.SlugField(max_length=300, blank=True)

    def save(self, *args, **kwargs):
        if not self.slug:
            # Slug field must be unique, so give it a temporary throw-away value
            temp_slug = uuid.uuid4().hex
            self.slug = temp_slug
            super(MyClass, self).save(*args, **kwargs)
            self.slug = "{}-{}".format(slugify(self.title), self.pk)
        super(MyClass, self).save(*args, **kwargs)

This has the advantage of only requiring a max of two DB writes per save call.

Answer 3

If You must have an counter inside slug, much better way will be:

class MyClass(models.Model):
    self.title = models.CharField()
    self.slug = models.SlugField(unique=True, blank=False)

    def save(self, *args, **kwargs):
        baseslug = slugify(self.title)
        try:
            last = MyClass.objects.filter(slug__regex="%s-\d+" % baseslug).latest('id')
            # safe, becouse slugify won't return regex special chars
            # also field with latest id will have highest counter in slug if it was populated that way

            trash, count = last.slug.rsplit('-', 1)            
            count = int(count)+1
        except:
            count = 1
        while count < MAXCOUNT:
            self.slug = "%s-%d" % (baseslug, count)
            try: 
                super(MyClass, self).save(*args, **kwargs)
                break
            except IntegrityError as exc:
                count += 1

it will query database only once to check what number put at the end of slug.