How to pass joined table value in view.php?

Hello,
I've created a blog and i want to pass a value from a joined table in view.php.
e.g.: I have "posts" and "categories" and it properly displays in index.php but when i click an individual article and trying to pass the same value there, i get "undefined index: category_name".

Any help with that?!

Thank you in advance!

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