Document and verify optional CMS associations

cms/cms.go

@@ -45,6 +45,8 @@ func (r Reference) Validate() error {
 
 // Associations are an immutable revision's term memberships and explicit links.
 // Reverse discovery reads the same links; callers must not store a second edge.
+// Terms and Links are independently optional. The zero value is valid for a
+// valid source; unclassified content never needs a placeholder taxonomy or link.
 type Associations struct {
 	Terms []string    `json:"terms,omitempty"`
 	Links []Reference `json:"links,omitempty"`

cms/cms_test.go

@@ -24,6 +24,20 @@ func TestAssociationsBoundsAndIdentity(t *testing.T) {
 		}
 	}
 }
+
+func TestAssociationsAreOptional(t *testing.T) {
+	for _, source := range []Reference{{Kind: "news", ID: "article"}, {Kind: "project", ID: "project"}, {Kind: "policy", ID: "terms"}} {
+		for _, raw := range []string{`{}`, `{"terms":null,"links":null}`, `{"terms":[],"links":[]}`, `{"terms":["go"]}`, `{"links":[{"kind":"project","id":"other"}]}`} {
+			var value Associations
+			if err := json.Unmarshal([]byte(raw), &value); err != nil {
+				t.Fatal(err)
+			}
+			if err := value.Validate(source); err != nil {
+				t.Fatalf("optional associations rejected: %s: %v", raw, err)
+			}
+		}
+	}
+}
 func TestTaxonomyAndTermText(t *testing.T) {
 	tax := Taxonomy{ID: "categories", Slug: "categories", Name: "Categories", Revision: 1, Active: true}
 	if tax.Validate() != nil {