Defer model validation to requested branches instead of entire model

pkg/report/model.go

@@ -108,13 +108,6 @@ func (m *model) Run(ctx context.Context) (err error) {
 			return errors.New("no model steps defined")
 		}
 
-		for _, s := range m.steps {
-			err = s.Validate()
-			if err != nil {
-				return err
-			}
-		}
-
 		return nil
 	}()
 	if err != nil {

pkg/report/model_graph.go

@@ -65,9 +65,30 @@ func (m *model) datasource(ctx context.Context, def *FrameDefinition) (ds Dataso
 		return nil, fmt.Errorf("unresolved source: %s", def.Source)
 	}
 
+	err = m.validateBranch(start)
+	if err != nil {
+		return
+	}
+
 	return m.reduceBranch(ctx, start)
 }
 
+func (m *model) validateBranch(n *modelGraphNode) (err error) {
+	err = n.step.Validate()
+	if err != nil {
+		return err
+	}
+
+	for _, c := range n.cc {
+		err = m.validateBranch(c)
+		if err != nil {
+			return err
+		}
+	}
+
+	return nil
+}
+
 func (m *model) reduceBranch(ctx context.Context, n *modelGraphNode) (out Datasource, err error) {
 	// leaf node, nothing else to do
 	if len(n.cc) == 0 {