Migrate or revert only some migrations

To only run the next two migrations:

rake db:migrate STEP=2

To revert the previous two migrations:

rake db:rollback STEP=2

To revert the last two migrations and then migrate to the most current version:

rake db:migrate:redo STEP=2

To migrate to a given migration number, regardless of whether that means migrating up or down:

rake db:migrate VERSION=20100627185630

To migrate exactly one individual migration out of the sequence* (careful):

rake db:migrate:up VERSION=20100627185630

To revert exactly one individual migration out of the sequence* (careful):

rake db:migrate:down VERSION=20100627185630

*) These migrations must (not) have been processed before (contained/not contained in the schema_migrations table) or nothing will happen.