Store info about two objects' relationship

I am still thinking about this, and I have come to a point where it is necessary.

I want something like this:

SetObjectLink(o1:Object,o2:Object,link:Object)
GetObjectLink:Object(o1:Object,o2:Object)

I am thinking a TMap with an intermediate type might be the way to go, but I don't know how I would sort the type.