Django：使用True True来增加Slug的大小

Question

I need to increase size of my slug to 500 which by default is 255 characters but same time I need to keep it unique as well. Is there a way around to make it happen?

models.py

class Product(models.Model):
    title = models.CharField(max_length=500)
    description = models.TextField(blank=True, null=True)
    price = models.DecimalField(max_digits=20, decimal_places=2)
    sku = models.CharField(null=True, max_length=100)
    url = models.URLField(blank=True)
    slug = models.SlugField(unique=True)

Please advise.

Answer 1

According to the documentation the maximum length for a SlugField is 255 characters. But it is the database backend that enforces it so, depending on which backend you are using, you might be able to increase that.

SQLite and PostgreSQL both allow max_length to be set to 500 with a UNIQUE constraint. Note that PostgreSQL will actually enforce the length limit, whereas SQLite doesn't really care and will allow you to insert strings greater than the declared size.

But the MySQL/Mariadb driver will enforce the 255 character limit in django/db/backends/mysql/validation.py . Within the standard backends, this is the only one that has this restriction. The 255 comes from the restriction that the maximum index key for a InnoDB column is 767 bytes. If you store UTF8 data in the column, as you must, it can take up to 3 bytes per character, which results in 767/3 = 255 characters.

One possible solution is to have an additional CharField containing a hash of the slug. That field will be short (eg 32 bytes for MD5 digest) and it will be unique which will stop duplicate slugs being inserted. Population of the slug and its hash can be done in Product.save() .

The model will be:

import hashlib
from django.utils.text import slugify

class Product(models.Model):
    title = models.CharField(max_length=500)
    description = models.TextField(blank=True, null=True)
    price = models.DecimalField(max_digits=20, decimal_places=2)
    sku = models.CharField(null=True, max_length=100)
    url = models.URLField(blank=True)
    slug = models.SlugField(max_length=500)
    slug_hash = models.CharField(max_length=32, unique=True)

    def save(self, *args, **kwargs):
        self.slug = slugify(self.title)
        self.slug_hash = hashlib.md5(self.slug).hexdigest()
        super(Product, self).save(*args, **kwargs)

Now if an attempt to save a record with the same slug is attempted an IntegrityError on the slug_hash will be raised.

Answer 2

Yes, it's possible using signals. The slug field will automatically get created/updated before saving the instance of the model. To make it unique you can concatenate the title with the id if a model instance with same slug already exists. See code bellow (add it to your model) :

def create_slug(instance, new_slug=None):
    """
    A function to create a slug
    if an object with slug already exists, it adds id value to slug
    else  just slugify the title
    """
    slug = slugify(instance.title)
    if new_slug is not None:
        slug = new_slug
    qs = Product.objects.filter(slug=slug).order_by('-id')
    exists = qs.exists()
    if exists:
        new_slug = "%s-%s" %(slug, qs.first().id)
        return create_slug(instance, new_slug=new_slug) #recursive call 
    return slug

def pre_save_product_receiver(sender, instance, *args, **kwargs):
    """
    A signal that calls the function create_slug if an object has not slug 
    """
    if not instance.slug:
        instance.slug = create_slug(instance)

pre_save.connect(pre_save_product_receiver, sender=Product)

To increase slug max characters replace your field with this :

slug = models.SlugField(max_length = 255, unique=True)

Don't go over 255 to avoid trouble in case you change your database technology.