Agile Web Development with Rails 5, 1st Edition

Creating and Running Migrations

A migration is simply a Ruby source file in your application’s db/migrate directory. Each migration file’s name starts with a number of digits (typically fourteen) and an underscore. Those digits are the key to migrations, because they define the sequence in which the migrations are applied—they are the individual migration’s version number.

The version number is the Coordinated Universal Time (UTC) timestamp at the time the migration was created. These numbers contain the four-digit year, followed by two digits each for the month, day, hour, minute, and second, all based on the mean solar time at the Royal Observatory in Greenwich, London. Because migrations tend to be created relatively infrequently and the ...