notes: clean up note-locked validation.

app/models/note.rb

@@ -8,7 +8,7 @@ class Note < ApplicationRecord
   validate :note_within_image
   after_save :update_post
   after_save :create_version
-  validate :post_must_not_be_note_locked
+  validate :validate_post_is_not_locked
 
   module SearchMethods
     def active
@@ -27,10 +27,8 @@ class Note < ApplicationRecord
 
   extend SearchMethods
 
-  def post_must_not_be_note_locked
-    if is_locked?
-      errors.add :post, "is note locked"
-    end
+  def validate_post_is_not_locked
+    errors[:post] << "is note locked" if post.is_note_locked?
   end
 
   def note_within_image
@@ -40,10 +38,6 @@ class Note < ApplicationRecord
     end
   end
 
-  def is_locked?
-    Post.exists?(["id = ? AND is_note_locked = ?", post_id, true])
-  end
-
   def rescale!(x_scale, y_scale)
     self.x *= x_scale
     self.y *= y_scale