class ActiveRecord::MigrationContext

the +target_version+.
If none of the conditions are met, +up+ will be run with

the +target_version+, then +down+ will be run.
If the +current_version+ in the schema is greater than

will be run.
0 then an empty array will be returned and no migrations
If the +current_version+ and +target_version+ are both

If +target_version+ is +nil+, +migrate+ will run +up+.

Runs the migrations in the +migrations_path+.

def migrate(target_version = nil, &block)
  case
  when target_version.nil?
    up(target_version, &block)
  when current_version == 0 && target_version == 0
    []
  when current_version > target_version
    down(target_version, &block)
  else
    up(target_version, &block)
  end
end