diff --git a/docs/content/1.guide/3.relationships/5.polymorphic.md b/docs/content/1.guide/3.relationships/5.polymorphic.md
index aaf37f35b..e2f7caf9e 100644
--- a/docs/content/1.guide/3.relationships/5.polymorphic.md
+++ b/docs/content/1.guide/3.relationships/5.polymorphic.md
@@ -231,6 +231,8 @@ class Tag extends Model {
 class Taggable extends Model {
   static entity = 'taggables'
 
+  static primaryKey = ['tag_id', 'taggable_id', 'taggable_type']
+
   static fields () {
     return {
       id: this.attr(null),