  A unified theory for the current through a nanoscale region of interacting
electrons connected to two leads which can be either ferromagnet or
superconductor is presented, yielding Meir-Wingreen-type formulas when applied
to specific circumstances. In such a formulation, the requirement of gauge
invariance for the current is satisfied automatically. Moreover, one can judge
unambiguously what quantities can be measured in the transport experiment.
