diff --git a/railties/doc/guides/source/form_helpers.txt b/railties/doc/guides/source/form_helpers.txt
index 7fb01bbf4863c..05f1b205dba62 100644
--- a/railties/doc/guides/source/form_helpers.txt
+++ b/railties/doc/guides/source/form_helpers.txt
@@ -403,6 +403,14 @@ As before, if you were to use `select` helper on a form builder scoped to `@pers
 <%= f.select(:city_id, ...) %>
 ----------------------------------------------------------------------------
 
+[WARNING] 
+=============================
+If you are using `select` (or similar helpers such as `collection_select`, `select_tag`) to set a `belongs_to` association you must pass the name of the foreign key (in the example above `city_id`) and not the name of association itself. Active Record will raise an error along the lines of
+--------
+ActiveRecord::AssociationTypeMismatch: City(#17815740) expected, got Fixnum(#1138750)
+--------
+when you pass the params hash to `Person.new` or `update_attributes` if you specify `city` instead of `city_id`. Another way of looking at this is that form helpers only edit attributes.
+============================
 Option tags from a collection of arbitrary objects
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~