i need custom database behaviour, but i dont know how to do it

I am working on a Categories application for my project. The model that i need to categorize ise Announces.. i descibed category in announces like this

class Category
    ...
    ...
    path = models.CharField ....

class Announce:
    ...
    ...
    ...
    category = models.ForeignKey(Category)

So my question,

This Announce model saves id of category on database and calls with id when it needed, but i want to make it save Category.path 

can i do it easly? or should i override something ? any ideas?

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