diff --git a/user_guide_src/source/models/model.rst b/user_guide_src/source/models/model.rst
index 6dbdb0a96951..67f104a268f6 100644
--- a/user_guide_src/source/models/model.rst
+++ b/user_guide_src/source/models/model.rst
@@ -332,6 +332,8 @@ of the columns in a ``$table``, while the array's values are the values to save
 
 .. literalinclude:: model/016.php
 
+.. important:: If the ``$primaryKey`` field is set to ``null`` then the update will affect all records in the table.
+
 Multiple records may be updated with a single call by passing an array of primary keys as the first parameter:
 
 .. literalinclude:: model/017.php