Add single_heading_group? helper at Budget::Group model

app/models/budget/group.rb

@@ -10,5 +10,8 @@ class Budget
     validates :name, presence: true, uniqueness: { scope: :budget }
     validates :slug, presence: true, format: /\A[a-z0-9\-_]+\z/
 
+    def single_heading_group?
+      headings.count == 1
+    end
   end
 end