Django: save() prohibited to prevent data loss due to unsaved related object 'predictions'

Welcome @OstapTelychko ! Just a side note here: When you’re posting code in the forum, surround that code between lines of three backtick - ` characters. This means you’ll have a line of ```, then your code, then another line of ```. This forces the forum software to keep your code properly formatted.
I’ve taken the liberty of modifying your post for this, please remember to do this yourself in the future.