If you get the message “No changes detected” after running:
python manage.py makemigrations MY_APP
AND you have seen all the Q&A on Stackoverflow related to first migrations or migrating from an older version AND none of that applies to you then may be I have the answer.
Maybe your model code has a bug. In my case, I had a complex and expensive calculation that I added as a @property of my model. Later, I decided to slightly de-normalize my database and actually make that property a database field. The problem was I forgot to delete the @property method. This was over-riding my new field. Which is why it was not detected. Duh…