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docs, link MySQL manual for multi column indexes. [ci ckip].

Closes #9131.
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1 parent 17b0edd commit 4e4a92e78a29d2508702336a440cb0d14cbf1f39 @senny senny committed with zzak Feb 4, 2014
Showing with 3 additions and 1 deletion.
  1. +3 −1 guides/source/active_record_validations.md
@@ -575,7 +575,9 @@ This helper validates that the attribute's value is unique right before the
object gets saved. It does not create a uniqueness constraint in the database,
so it may happen that two different database connections create two records
with the same value for a column that you intend to be unique. To avoid that,
-you must create a unique index in your database.
+you must create a unique index on both columns in your database. See
+[the MySQL manual](http://dev.mysql.com/doc/refman/5.6/en/multiple-column-indexes.html)
+for more details about multi column indexes.
```ruby
class Account < ActiveRecord::Base

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