mosfet - How does the signal change cause a current change in this circuit?

Consider a Common Source NMOS amplifier with current source as a load.

^{simulate this circuit – Schematic created using CircuitLab}

If the drain current is constant as dictated by the current source, how could Vin change the drain current of the transistor?

EDIT:

If there is a current source load connected to the drain of a CS amplifier, would the current be determined by the current source alone?