OS X sandboxed apps and symlinks problem - solution
On OS X versions that support sandboxed apps (beginning from Lion), it's sometimes impossible to symlink a sandboxed app application support files to other partition, at least, an inner portion of the folder structure.
For example, Airmail is a sandboxed email client. I'd like to move its settings and email db to another partition by symlinking it, but it doesn't work with the app if I symlink an inner folder of its library container:
cd ~/Library/Containers/it.bloop.airmail.beta7/Data/Library/Application\ Support/ ln -s /Volumes/doxx/sys/Airmail Airmail
The solution is to symlink the whole container:
cd ~/Library/Containers/ ln -s /Volumes/doxx/sys/Airmail/it.bloop.airmail it.bloop.airmail