rails migration 有一个change_column_default方法，用于改变数据库的默认值,之前不知道，我用的是change_column

http://api.rubyonrails.org/v2.3.8/classes/ActiveRecord/ConnectionAdapters/SchemaStatements.html#M001436

change_column_default(table_name, column_name, default)

Sets a new default value for a column. If you want to set the default value to NULL, you are out of luck. You need to DatabaseStatements#execute the appropriate SQL statement yourself.

Examples

 change_column_default(:suppliers, :qualification, 'new')
 change_column_default(:accounts, :authorized, 1)