What is the test for the use of a motion of common law common law for unjust enrichment?

Furthermore, the mere fact that the matter pleaded is novel or unusual does not militate against its use, at least at this stage of the proceedings. In that respect, the comments of Dickson J. in Pettkus v. Becker (1980), 1980 CanLII 22 (SCC), 117 D.L.R. (3d) 257 (S.C.C) ("Pettkus") at 273 are most instructive: It would be undesirable and indeed impossible to attempt to define all the circumstances in which an unjust enrichment might arise. The great advantage of the ancient principles of equity is their flexibility: the judiciary is thus able to shape these malleable principles so as to accommodate the changing needs and mores of society, in order to achieve justice.