Often if we define a model object, we will access a page related to that object by making a HTTP request that contains the primary key, the slug, or even both.
A problem often arises if objects are removed or update. People often want to avoid "gaps" between primary keys, and thus update the primary keys that are greater than the one that is removed.
Furthermore if the object itself changes, often developers want to update the slug accordingly. So if for example a
Blog model contains a title, and the slug is derived from that title, then people want to update the slug accordingly.
Why is it a problem?
An URI is a Uniform Resource Identifier, this thus means that with such a URI, we can find the data about the object we are looking for.
Often people will bookmark a page in the browser, send a URI in an email to a colleague, and search engines might still point to a page that no longer exists.
In 1998, Tim Berners-Lee wrote a document named Cool URIs don't change [w3.org]. In this document, Berners-Lee explains that changing URI's is not a good idea, and further discusses typical excuses to change an URI.
What can be done to resolve the problem?
Once the slug and the primary key are set, one usually should not alter these. If the slug depends on the title, you thus set the slug based on the first value of the title.
Making no changes to the slug and/or primary key, is however not enough, since the
urls.py will determine the URL as well.
If you really need to make a change to the slug and/or primary key, then you should implement (permanent) redirects (HTTP 301 [wiki]) from the old URL to the new URL. This thus likely means that you need to make a model that translates "old" slugs to new slugs. Since slugs should be unique it also means that you should check if the slug for a new model object is already an old slug.