allow to force NilVersion

cli/commands.go

@@ -42,7 +42,7 @@ func dropCmd(m *migrate.Migrate) {
 	}
 }
 
-func forceCmd(m *migrate.Migrate, v uint) {
+func forceCmd(m *migrate.Migrate, v int) {
 	if err := m.Force(v); err != nil {
 		log.fatalErr(err)
 	}

cli/main.go

@@ -182,12 +182,16 @@ Commands:
 			log.fatal("error: please specify version argument V")
 		}
 
-		v, err := strconv.ParseUint(flag.Arg(1), 10, 64)
+		v, err := strconv.ParseInt(flag.Arg(1), 10, 64)
 		if err != nil {
 			log.fatal("error: can't read version argument V")
 		}
 
-		forceCmd(migrater, uint(v))
+		if v < -1 {
+			log.fatal("error: argument V must be >= -1")
+		}
+
+		forceCmd(migrater, int(v))
 
 		if log.verbose {
 			log.Println("Finished after", time.Now().Sub(startTime))

database/driver.go

@@ -63,6 +63,7 @@ type Driver interface {
 
 	// SetVersion saves version and dirty state.
 	// Migrate will call this function before and after each call to Run.
+	// version must be >= -1. -1 means NilVersion.
 	SetVersion(version int, dirty bool) error
 
 	// Version returns the currently active version and if the database is dirty.

migrate.go

@@ -355,12 +355,16 @@ func (m *Migrate) Run(migration ...*Migration) error {
 // Force sets a migration version.
 // It does not check any currently active version in database.
 // It does not check if the database is dirty.
-func (m *Migrate) Force(version uint) error {
+func (m *Migrate) Force(version int) error {
+	if version < -1 {
+		panic("version must be >= -1")
+	}
+
 	if err := m.lock(); err != nil {
 		return err
 	}
 
-	if err := m.databaseDrv.SetVersion(int(version), false); err != nil {
+	if err := m.databaseDrv.SetVersion(version, false); err != nil {
 		return m.unlockErr(err)
 	}