diff --git a/guides/source/association_basics.md b/guides/source/association_basics.md
index fb02ae0a4805c..458f3839a6b41 100644
--- a/guides/source/association_basics.md
+++ b/guides/source/association_basics.md
@@ -125,7 +125,7 @@ end
 When used alone, `belongs_to` produces a one-directional one-to-one connection. Therefore each book in the above example "knows" its author, but the authors don't know about their books.
 To setup a [bi-directional association](#bi-directional-associations) - use `belongs_to` in combination with a `has_one` or `has_many` on the other model.
 
-`belongs_to` does not ensure reference consistency, so depending on the use case, you might also need to add a database-level foreign key constraint on the reference column, like this:
+`belongs_to` does not ensure reference consistency if `optional` is set to true, so depending on the use case, you might also need to add a database-level foreign key constraint on the reference column, like this:
 
 ```ruby
 create_table :books do |t|