this configuration is also called Widlar current source, you can search "Widlar current source" in Wiki.
and this is also a voltage independent current reference in which you need to add the start-up circuit to make it work right.
the resistor is added due to this equation Vgs=sqrt(2IdsL/unCoxW)+Vth, the wider the transistor, the smaller the Vgs, so you need the transistor to make up the voltage drop.