struct CollectionElement : public Moveable<CollectionElement>
{
   String unique_id;
   Value data; // Whatever
};

struct Referer : public Moveable<Referer>
{
   String unique_id;
   Value data; // Whatever
   String collection_id;
}

Index<CollectionElement> collection;
Vector<Referer> ref;

int FindRefCollection(Referer &ref) {
   unsigned collection_hash = GetHashValue(ref.collection_id);
   // Now we need to find the CollectionElement by hash only
   // but the closest you can get is Find(T &x, unsigned hash)
   return -1;
}