Show validation error when trying to attach BookImages to an empty/nonexistent Book.

[PaperNick]

Attempts to solve #2

[PaperNick]

Hey @philgyford, that's my attempt to solve this issue.

I'm not sure about the accuracy of the helper functions is_empty_form and is_form_persisted. Do you know about any built-in methods which do something similar?

[philgyford]

Thanks!

I'm pretty sure I'm making a pig's ear of making some edits to this (I'm not used to handling PRs, sorry) and have made some tweaks on the https://github.com/philgyford/django-nested-inline-formsets-example/tree/PaperNick-fix/add-validation-when-inserting-nested-inlines-in-empty-parent-form branch.

I've mainly added lots of comments to help explain what's going on to others. And also because I find it hard to follow form logic at the best of times. I also made some of the code slightly more verbose because I find it hard to parse lines like return form.is_valid() and not form.cleaned_data in one go. Hopefully my explanatory comments are correct?

I don't know of any built-in methods that do similar to your new methods, but I haven't worked with forms for some time. It seemed a bit odd to have a is_empty_form function but only use it as not is_empty_form()... but I couldn't think what a concise name for its opposite meaning would be!

[PaperNick]

Thanks for the edits! Your comments are completely fine. I cherry-picked your commit and pushed it here.

About the is_empty_form, you have a point but I couldn't think of any other name as well.

[PaperNick]

By the way, if you want to make more edits to my PR, you can push directly in my branch.

[philgyford]

Thanks for all of that!

Years of usually being the only developer on a project, or the only one on particular parts of the code, mean my skills at handling PRs are lacking!

[PaperNick]

Thank you as well! Because of your repository, I was able to quickly understand the logic behind the nested inline formsets and get my job done. Your explanatory comments were a huge help.