When you select the wrong iPhoto library–say a backup copy of your iPhoto library–and upload your son’s birthday pictures, and then erase your memory card to take more pictures, you find yourself in a pile of hurt.
It is unclear if an rsync between both libraries will work: will the extra photos appear in the library or not. Will they have an event attached, etc. Your main library with over 5 years of photos of your child is not something to experiment with.
So I figured that you need to find out the files that are present in one library and not in the other, and copy them to an outside location. From that point on, you can just import the photos using the standard iPhoto tooling into the correct library.
Now my NAS (a Synology) creates additional files and folders, and OS X is also notorious for adding hidden folders. I particularly wanted to exclude those files and folders as they were not instrumental in re-importing the missing photographs.
Fortunately rsync has an option to compare two folders and synchronize the missing or updated files to a third location. The command below will copy over the missing or updated files in my master iPhoto library from my backup iPhoto library to an external folder.
Note the exclusion of the “CR2” files–I exclude the RAW versions of my photographs because I didn’t want to have to edit 250 RAW pictures. The JPGs will be good enough for this birthday party. If you care about your RAWs then remove that line.
I can then import these photos from the /volume1/Photos/2014 folder into my master iPhoto library.
Of course note that this is provided as is and you should check if this works for your usecase before running the script on your own photo libraries. Use at your own risk!