Custom field to Django, storing the hashed value of some other field.

MySQL doesn't allow to create indexes on text fields, whose length is greater than 255 ([or something like that] (https://lmgtfy.com/?q=mysql+255+index)). This can be a practical problem if you want to store a list of unique urls in your database (accepted length of url is up to 2083 characters).

The good solution to this problem is to create two columns in your database:

1. column of length 2083, storing the raw url address
2. column of length 40, storing hashed value of url

Then create an index on the second column. Class HashField implements a custom field in Django, which autoamtically stores the hashed value of the original field (such as url field) everytime the model is saved.

Using HashField, you can store the list of unique urls in Django as follows:

class VisitedSite(models.Model):
  url = models.URLField(max_length=2083)
  url_hash = HashField(original='url', unique=True)

  def __unicode__(self):
    return self.url
...

site = VisitedSite(url='https://github.com')
site.save() # url_hash is automatically computed and saved to database