iPad Midi keyboard options - lightning

I have a mini retina that I'd like to play with some more synths on. I'd like to pick up a small midi keyboard to use with it, but can't find many good options. I was hoping that someone had come out with a bluetooth midi implementation by this point, but that doesn't seem to be out there. What other options do I have? I'd love something that I can connect directly - can I just use my lightnight - USB cable to connect directly to a usb keyboard? It seems that I can't, but I don't really understand why. I'd appreciate any help.