Doc SchemaMigration class.

1 files changed, 5 insertions, 1 deletions

diff --git a/activerecord/lib/active_record/schema_migration.rb b/activerecord/lib/active_record/schema_migration.rb
index cb47bf23f7..b384529e75 100644
--- a/activerecord/lib/active_record/schema_migration.rb
+++ b/activerecord/lib/active_record/schema_migration.rb
@@ -2,7 +2,11 @@ require 'active_record/scoping/default'
 require 'active_record/scoping/named'
 
 module ActiveRecord
-  class SchemaMigration < ActiveRecord::Base
+  # This class is used to create a table that keeps track of which migrations
+  # have been applied to a given database. When a migration is run, its schema
+  # number is inserted in to the `SchemaMigration.table_name` so it doesn't need
+  # to be executed the next time.
+  class SchemaMigration < ActiveRecord::Base # :nodoc:
     class << self
       def primary_key
         nil