[Fixed]-What is the correct way to auto-create related model objects in django-registration?

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cleaned_data is the dictionary of data after all validation in every field in that form. Now you can decide whether to rely on it or not(preferably you should).So as a pseudocode we could say cleaned_data + errors will be all fields.
commit is used to decide whether it should commit to db(write). From the above code, to add a related model object like profile, the original object(User) has to be created first.Thats why it make force commit.
To add a related object on object creation, there are multiple ways like post_save signals, override model save, override form save etc.So you are using one of good approach, I would say.