Implemented the Rollback feature.

In most dev/testing environments it is nice to have a way to reset/rollback
the entire database. This commit adds this feature in the most basic form;
when Rollback() is called, it will rollback ALL migrations that have run
leaving the database is a mostly pristine state.

There are a few potential issues here. For starters, devs may not always
want all migrations to be rolled back. The only current fix is to course
in this case is to create a migrator that only contains migrations they
want rolled back.

Another potential issue is that any migration that doesn't provide a
Rollback function will not be rolled back, but there also won't be any
errors. A message is printed out when this happens to help avoid some
confusion, but I could see this still causing issues. I'm not 100% sure
what the best long term solution is, but feel the current version is
good enough to move forward since it satisfies my needs.

sqlx.go

@@ -49,6 +49,41 @@ func (s *Sqlx) Migrate(sqlDB *sql.DB, dialect string) error {
 	return nil
 }
 
+// Rollback will run all rollbacks using the provided db connection.
+func (s *Sqlx) Rollback(sqlDB *sql.DB, dialect string) error {
+	db := sqlx.NewDb(sqlDB, dialect)
+
+	s.printf("Creating/checking migrations table...\n")
+	err := s.createMigrationTable(db)
+	if err != nil {
+		return err
+	}
+	for i := len(s.Migrations) - 1; i >= 0; i-- {
+		m := s.Migrations[i]
+		if m.Rollback == nil {
+			s.printf("Rollback not provided: %v\n", m.ID)
+			continue
+		}
+		var found string
+		err := db.Get(&found, "SELECT id FROM migrations WHERE id=$1", m.ID)
+		switch err {
+		case sql.ErrNoRows:
+			s.printf("Skipping rollback: %v\n", m.ID)
+			continue
+		case nil:
+			s.printf("Running rollback: %v\n", m.ID)
+			// we need to run the rollback so we continue to code below
+		default:
+			return fmt.Errorf("looking up rollback by id: %w", err)
+		}
+		err = s.runRollback(db, m)
+		if err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
 func (s *Sqlx) printf(format string, a ...interface{}) (n int, err error) {
 	printf := s.Printf
 	if printf == nil {
@@ -89,6 +124,30 @@ func (s *Sqlx) runMigration(db *sqlx.DB, m SqlxMigration) error {
 	return nil
 }
 
+func (s *Sqlx) runRollback(db *sqlx.DB, m SqlxMigration) error {
+	errorf := func(err error) error { return fmt.Errorf("running rollback: %w", err) }
+
+	tx, err := db.Beginx()
+	if err != nil {
+		return errorf(err)
+	}
+	_, err = db.Exec("DELETE FROM migrations WHERE id=$1", m.ID)
+	if err != nil {
+		tx.Rollback()
+		return errorf(err)
+	}
+	err = m.Rollback(tx)
+	if err != nil {
+		tx.Rollback()
+		return errorf(err)
+	}
+	err = tx.Commit()
+	if err != nil {
+		return errorf(err)
+	}
+	return nil
+}
+
 // SqlxMigration is a unique ID plus a function that uses a sqlx transaction
 // to perform a database migration step.
 //
@@ -97,24 +156,37 @@ func (s *Sqlx) runMigration(db *sqlx.DB, m SqlxMigration) error {
 type SqlxMigration struct {
 	ID       string
 	Migrate  func(tx *sqlx.Tx) error
+	Rollback func(tx *sqlx.Tx) error
 }
 
 // SqlxQueryMigration will create a SqlxMigration using the provided id and
 // query string. It is a helper function designed to simplify the process of
 // creating migrations that only depending on a SQL query string.
-func SqlxQueryMigration(id, query string) SqlxMigration {
-	m := SqlxMigration{
-		ID: id,
-		Migrate: func(tx *sqlx.Tx) error {
+func SqlxQueryMigration(id, upQuery, downQuery string) SqlxMigration {
+	queryFn := func(query string) func(tx *sqlx.Tx) error {
+		if query == "" {
+			return nil
+		}
+		return func(tx *sqlx.Tx) error {
 			_, err := tx.Exec(query)
 			return err
-		},
+		}
+	}
+
+	m := SqlxMigration{
+		ID:       id,
+		Migrate:  queryFn(upQuery),
+		Rollback: queryFn(downQuery),
 	}
 	return m
 }
 
 // SqlxFileMigration will create a SqlxMigration using the provided file.
-func SqlxFileMigration(id, filename string) SqlxMigration {
+func SqlxFileMigration(id, upFile, downFile string) SqlxMigration {
+	fileFn := func(filename string) func(tx *sqlx.Tx) error {
+		if filename == "" {
+			return nil
+		}
 		f, err := os.Open(filename)
 		if err != nil {
 			// We could return a migration that errors when the migration is run, but I
@@ -125,12 +197,16 @@ func SqlxFileMigration(id, filename string) SqlxMigration {
 		if err != nil {
 			panic(err)
 		}
-	m := SqlxMigration{
-		ID: id,
-		Migrate: func(tx *sqlx.Tx) error {
+		return func(tx *sqlx.Tx) error {
 			_, err := tx.Exec(string(fileBytes))
 			return err
-		},
+		}
+	}
+
+	m := SqlxMigration{
+		ID:       id,
+		Migrate:  fileFn(upFile),
+		Rollback: fileFn(downFile),
 	}
 	return m
 }

sqlx_test.go

@@ -33,7 +33,7 @@ func TestSqlx(t *testing.T) {
 				return 0, nil
 			},
 			Migrations: []migrate.SqlxMigration{
-				migrate.SqlxQueryMigration("001_create_courses", createCoursesSql),
+				migrate.SqlxQueryMigration("001_create_courses", createCoursesSql, ""),
 			},
 		}
 		err := migrator.Migrate(db, "sqlite3")
@@ -54,7 +54,7 @@ func TestSqlx(t *testing.T) {
 				return 0, nil
 			},
 			Migrations: []migrate.SqlxMigration{
-				migrate.SqlxQueryMigration("001_create_courses", createCoursesSql),
+				migrate.SqlxQueryMigration("001_create_courses", createCoursesSql, ""),
 			},
 		}
 		err := migrator.Migrate(db, "sqlite3")
@@ -73,8 +73,8 @@ func TestSqlx(t *testing.T) {
 				return 0, nil
 			},
 			Migrations: []migrate.SqlxMigration{
-				migrate.SqlxQueryMigration("001_create_courses", createCoursesSql),
-				migrate.SqlxQueryMigration("002_create_users", createUsersSql),
+				migrate.SqlxQueryMigration("001_create_courses", createCoursesSql, ""),
+				migrate.SqlxQueryMigration("002_create_users", createUsersSql, ""),
 			},
 		}
 		err = migrator.Migrate(db, "sqlite3")
@@ -95,7 +95,7 @@ func TestSqlx(t *testing.T) {
 				return 0, nil
 			},
 			Migrations: []migrate.SqlxMigration{
-				migrate.SqlxFileMigration("001_create_widgets", "testdata/widgets.sql"),
+				migrate.SqlxFileMigration("001_create_widgets", "testdata/widgets.sql", ""),
 			},
 		}
 		err := migrator.Migrate(db, "sqlite3")
@@ -107,16 +107,68 @@ func TestSqlx(t *testing.T) {
 			t.Fatalf("db.Exec() err = %v; want nil", err)
 		}
 	})
+
+	t.Run("rollback", func(t *testing.T) {
+		db := sqliteInMem(t)
+		migrator := migrate.Sqlx{
+			Printf: func(format string, args ...interface{}) (int, error) {
+				t.Logf(format, args...)
+				return 0, nil
+			},
+			Migrations: []migrate.SqlxMigration{
+				migrate.SqlxQueryMigration("001_create_courses", createCoursesSql, dropCoursesSql),
+			},
+		}
+		err := migrator.Migrate(db, "sqlite3")
+		if err != nil {
+			t.Fatalf("Migrate() err = %v; want nil", err)
+		}
+		_, err = db.Exec("INSERT INTO courses (name) VALUES ($1) ", "cor_test")
+		if err != nil {
+			t.Fatalf("db.Exec() err = %v; want nil", err)
+		}
+		err = migrator.Rollback(db, "sqlite3")
+		if err != nil {
+			t.Fatalf("Rollback() err = %v; want nil", err)
+		}
+		var count int
+		err = db.QueryRow("SELECT COUNT(id) FROM courses;").Scan(&count)
+		if err == nil {
+			// Want an error here
+			t.Fatalf("db.QueryRow() err = nil; want table missing error")
+		}
+		// Don't want to test inner workings of lib, so let's just migrate again and verify we have a table now
+		err = migrator.Migrate(db, "sqlite3")
+		if err != nil {
+			t.Fatalf("Migrate() err = %v; want nil", err)
+		}
+		_, err = db.Exec("INSERT INTO courses (name) VALUES ($1) ", "cor_test")
+		if err != nil {
+			t.Fatalf("db.Exec() err = %v; want nil", err)
+		}
+		err = db.QueryRow("SELECT COUNT(*) FROM courses;").Scan(&count)
+		if err != nil {
+			// Want an error here
+			t.Fatalf("db.QueryRow() err = %v; want nil", err)
+		}
+		if count != 1 {
+			t.Fatalf("count = %d; want %d", count, 1)
+		}
+	})
 }
 
-var createCoursesSql = `
+var (
+	createCoursesSql = `
 CREATE TABLE courses (
   id serial PRIMARY KEY,
   name text
 );`
+	dropCoursesSql = `DROP TABLE courses;`
 
-var createUsersSql = `
+	createUsersSql = `
 CREATE TABLE users (
   id serial PRIMARY KEY,
   email text UNIQUE NOT NULL
 );`
+	dropUsersSql = `DROP TABLE users;`
+)

testdata/widgets.down.sql

@@ -0,0 +1,2 @@
+DROP TABLE widgets;
+