common: handle unordered relationships correctly

Continued part of #198 after @JasonFromSpace pointed out
an error with the original fix.

common/relationships.go

@@ -67,10 +67,16 @@ func (r Relationships) AddRelationship(target, ctype string) Relationship {
 	}
 	rel := relationships.NewRelationship()
 	nextID := len(r.x.Relationship) + 1
+	used := map[string]struct{}{}
+
+	// identify IDs in  use
 	for _, exRel := range r.x.Relationship {
-		if exRel.IdAttr == fmt.Sprintf("rId%d", nextID) {
-			nextID++
+		used[exRel.IdAttr] = struct{}{}
 	}
+	// find the next ID that is unused
+	for _, ok := used[fmt.Sprintf("rId%d", nextID)]; ok; _, ok = used[fmt.Sprintf("rId%d", nextID)] {
+		nextID++
+
 	}
 	rel.IdAttr = fmt.Sprintf("rId%d", nextID)
 	rel.TargetAttr = target

common/relationships_test.go

@@ -65,3 +65,27 @@ func TestRelationshipsCreation(t *testing.T) {
 		t.Errorf("expected %s, got %s", exp, rel.ID())
 	}
 }
+
+func TestRelationshipsCreationUnordered(t *testing.T) {
+	r := common.NewRelationships()
+	r.AddRelationship("foo", "http://bar").X().IdAttr = "rId1"
+	r.AddRelationship("foo", "http://bar").X().IdAttr = "rId3"
+	r.AddRelationship("foo", "http://bar").X().IdAttr = "rId5"
+	r.AddRelationship("foo", "http://bar").X().IdAttr = "rId7"
+
+	// len is 4, but id 5 is used, so we should get 6 next
+	rel := r.AddRelationship("foo", "http://bar")
+
+	exp := "rId6"
+	if rel.ID() != exp {
+		t.Errorf("expected %s, got %s", exp, rel.ID())
+	}
+
+	// would get 7, but it's in use so we get 8 now
+	rel = r.AddRelationship("foo", "http://bar")
+	exp = "rId8"
+	if rel.ID() != exp {
+		t.Errorf("expected %s, got %s", exp, rel.ID())
+	}
+
+}