This problem has been an issue for many users, and is the topic of several long threads, like this one. Apple had many useful features in iPhoto (such as copy file names to photo titles, which it did automatically). Most of those features were removed from Photos, and replaced with less useful ones. For example, I want to sort by photo title, which is the same as the file name. Photo does not allow me to copy file names to titles, and only sorts by file date or the date the picture was taken. I have 14,000 photos from film cameras; the photo date does not exist, so Photos sorts by scan date -- less than useless. Users have created scripts, such as the one a few messages ago, that does this job -- it takes about ten lines of code. It would seem an easy feature to add to Photos, but there is no evidence that it has any priority at all. The main reason why I upgraded to El Capitan, is that these discussion threads suggested that Apple would upgrade Photos to include sort options. Well, I upgraded, and no sort options exist.
Copying file names to title names, and providing sort options is not "professional" use, it's basic use. I am grateful to Apple for providing this for free, and would be willing to pay for sorting options, which were included in iPhoto for free. Charge money if you have to, but we really do need a suite of sort options.
This is why this thread is 13 pages long! With all due respect to Apple, sometimes it seems like valid and widespread user needs and suggestions receive no attention for months, years, or perhaps ever. It would not seem difficult to include a 10-20 line script to end these discussions. It's a low-hanging fruit....
Personally, if Photos included a set of sort options, including a "light table" that allows a matrix of photos to be dragged to better locations, and that sort list be used for future "slide shows", Photos would receive 5+++ stars from me. Without it, it's much less. The best option is for anyone interested in this problem to send a feedback note to Apple requesting these sort options. The more feedback, the better -- and maybe, maybe we'll get what iPhoto had.