- To determine the current version, the migrate task looks in your database for a table called "schema_info", and reads the value of the "version" field from it. If the table is not there, or is empty, then version is nil, and nil.to_i = 0.
- To determine the target version, the task gets the highest-numbered file from the db/migrate directory and parses an integer out of the filename.
- You can override the target version by passing in a command-line parameter like so:
rake db:migrate VERSION=XXX
So, as the famous "outraged of Tunbridge Wells" might say, why oh why oh why was my migration task always reverting back to previous versions, despite having migration files numbered sequentially right the way up to version 19 (and counting) ?
In vain did I bang my head against the desk, curse migrations and all they stood for, implore the heavens to rain down brimstone upon the head of DHH and all his acolytes, and impugn the parentage of my PC, Windows, Oracle and databases in general....
On a trawl through the Railties source code, it turns out there's a third, more surreptitious way to tell the migrate task which version you'd like to migrate to - if there's an environment variable called VERSION, it will take the value from that. And when I looked at my System Variables in XP, what did I find?