django how to change fields input according to related model field

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Python

Django

I'm trying to make a code-name(*i.e: Test Post will be TP-1, TP-2 ...*) thing on Django. There is **news** model, and **post** model i got. When user create news title hence post's codename will be according it's news's title. For example the

`news.title='Test News'`

then assume that I created a first post and assign it in variable **a**

`print a.codename` must return `TN-1`

Currently i can take News title by it's abbreviation in 2 or 3 letter and problem is how to add number after letters. And Posts are **related** to News by `ForeignKey` therefore number should increase according news like shown below.

* Test news
1. TN-1
2. TN-2
* Foo news
1. FN-1
2. FN-3
3. FN-4