Warn users about the race condition in validates_uniqueness_of. [Koz]

git-svn-id: http://svn-commit.rubyonrails.org/rails/trunk@8250 5ecf4fe2-1ee6-0310-87b1-e25e094e27de

1 files changed, 4 insertions, 0 deletions

diff --git a/activerecord/lib/active_record/validations.rb b/activerecord/lib/active_record/validations.rb
index bad07dd5c4..c6e6d24c91 100755
--- a/activerecord/lib/active_record/validations.rb
+++ b/activerecord/lib/active_record/validations.rb
@@ -595,6 +595,10 @@ module ActiveRecord
       # When the record is created, a check is performed to make sure that no record exists in the database with the given value for the specified
       # attribute (that maps to a column). When the record is updated, the same check is made but disregarding the record itself.
       #
+      # Because this check is performed outside the database there is still a chance that duplicate values
+      # will be inserted in two parrallel transactions.  To guarantee against this you should create a 
+      # unique index on the field. See +create_index+ for more information.
+      #
       # Configuration options:
       # * <tt>message</tt> - Specifies a custom error message (default is: "has already been taken")
       # * <tt>scope</tt> - One or more columns by which to limit the scope of the uniquness constraint.