Creating and Running Migrations

Migrations in depth

Migration is simply a Ruby source file in our 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 of this is based on the mean solar time at the Royal Observatory in Greenwich, London. Since migrations tend to be created relatively infrequently and the accuracy is recorded down to the second, the chances of any two people getting the same timestamp are vanishingly small. The benefit of having timestamps that can be deterministically ordered far outweighs the minuscule risk of this occurring.

Here’s what the db/migrate directory of our Depot application looks like:

depot> ls

...