Conditional migrations

[nbozic-zz]

Problem is add another column, move data and then drop column. If you add 3 migrations for this and if you configured application level to work async when application starts, you have timing issue when column should be dropped. Also if you add drop migration to SCHEME migration later on then someone who has clean slate or did not upgrade recently, will not issue DATA migration since configuration is as follows:

• SCHEME migration starts
• when SCHEME migration is done start application
• fire DATA migration ASYNC