Clarify the validation of present associations

I think that it is not clear what means that _an association is
present_. Add that it is checking that the foreign key is not empty and
that the referenced object exists to clarify it.

1 files changed, 2 insertions, 1 deletions

diff --git a/guides/source/active_record_validations.md b/guides/source/active_record_validations.md
index afe837a97c..25f6f7bd2c 100644
--- a/guides/source/active_record_validations.md
+++ b/guides/source/active_record_validations.md
@@ -538,7 +538,8 @@ end
 
 If you want to be sure that an association is present, you'll need to test
 whether the associated object itself is present, and not the foreign key used
-to map the association.
+to map the association. This way, it is not only checked that the foreign key
+is not empty but also that the referenced object exists.
 
 ```ruby
 class LineItem < ApplicationRecord