Add dump command (#23)

Adds `dbmate dump` command to write the database schema to a file.

The intent is for this file to be checked in to the codebase, similar to Rails' `schema.rb` (or `structure.sql`) file. This allows developers to share a single file documenting the database schema, and makes it considerably easier to review PRs which add (or change) migrations.

The existing `up`, `migrate`, and `rollback` commands will automatically trigger a schema dump, unless `--no-dump-schema` is passed.

Closes https://github.com/amacneil/dbmate/issues/5

.gitignore

@@ -1,2 +1,3 @@
-/dist
 .DS_Store
+/dist
+/testdata/db/schema.sql

Dockerfile

@@ -4,6 +4,14 @@ FROM golang:1.9 as build
 # required to force cgo (for sqlite driver) with cross compile
 ENV CGO_ENABLED 1
 
+# install database clients
+RUN apt-get update \
+	&& apt-get install -y --no-install-recommends \
+		mysql-client \
+		postgresql-client \
+		sqlite3 \
+	&& rm -rf /var/lib/apt/lists/*
+
 # development dependencies
 RUN go get \
 	github.com/golang/lint/golint \

cmd/dbmate/main.go

@@ -30,15 +30,24 @@ func NewApp() *cli.App {
 	app.Version = dbmate.Version
 
 	app.Flags = []cli.Flag{
+		cli.StringFlag{
+			Name:  "env, e",
+			Value: "DATABASE_URL",
+			Usage: "specify an environment variable containing the database URL",
+		},
 		cli.StringFlag{
 			Name:  "migrations-dir, d",
 			Value: dbmate.DefaultMigrationsDir,
 			Usage: "specify the directory containing migration files",
 		},
 		cli.StringFlag{
-			Name:  "env, e",
-			Value: "DATABASE_URL",
-			Usage: "specify an environment variable containing the database URL",
+			Name:  "schema-file, s",
+			Value: dbmate.DefaultSchemaFile,
+			Usage: "specify the schema file location",
+		},
+		cli.BoolFlag{
+			Name:  "no-dump-schema",
+			Usage: "don't update the schema file on migrate/rollback",
 		},
 	}
 
@@ -49,14 +58,14 @@ func NewApp() *cli.App {
 			Usage:   "Generate a new migration file",
 			Action: action(func(db *dbmate.DB, c *cli.Context) error {
 				name := c.Args().First()
-				return db.New(name)
+				return db.NewMigration(name)
 			}),
 		},
 		{
 			Name:  "up",
 			Usage: "Create database (if necessary) and migrate to the latest version",
 			Action: action(func(db *dbmate.DB, c *cli.Context) error {
-				return db.Up()
+				return db.CreateAndMigrate()
 			}),
 		},
 		{
@@ -88,6 +97,13 @@ func NewApp() *cli.App {
 				return db.Rollback()
 			}),
 		},
+		{
+			Name:  "dump",
+			Usage: "Write the database schema to disk",
+			Action: action(func(db *dbmate.DB, c *cli.Context) error {
+				return db.DumpSchema()
+			}),
+		},
 	}
 
 	return app
@@ -111,8 +127,10 @@ func action(f func(*dbmate.DB, *cli.Context) error) cli.ActionFunc {
 		if err != nil {
 			return err
 		}
-		db := dbmate.NewDB(u)
+		db := dbmate.New(u)
+		db.AutoDumpSchema = !c.GlobalBool("no-dump-schema")
 		db.MigrationsDir = c.GlobalString("migrations-dir")
+		db.SchemaFile = c.GlobalString("schema-file")
 
 		return f(db, c)
 	}

docker-compose.yml

@@ -13,13 +13,13 @@ services:
         condition: service_healthy
 
   mysql:
-    image: mysql
+    image: mysql:5.7
     environment:
       MYSQL_ROOT_PASSWORD: root
     healthcheck:
       test: ["CMD", "mysqladmin", "status", "-proot"]
 
   postgres:
-    image: postgres
+    image: postgres:9.6
     healthcheck:
       test: ["CMD", "pg_isready"]

pkg/dbmate/db.go

@@ -15,17 +15,24 @@ import (
 // DefaultMigrationsDir specifies default directory to find migration files
 var DefaultMigrationsDir = "./db/migrations"
 
+// DefaultSchemaFile specifies default location for schema.sql
+var DefaultSchemaFile = "./db/schema.sql"
+
 // DB allows dbmate actions to be performed on a specified database
 type DB struct {
+	AutoDumpSchema bool
 	DatabaseURL    *url.URL
 	MigrationsDir  string
+	SchemaFile     string
 }
 
-// NewDB initializes a new dbmate database
-func NewDB(databaseURL *url.URL) *DB {
+// New initializes a new dbmate database
+func New(databaseURL *url.URL) *DB {
 	return &DB{
+		AutoDumpSchema: true,
 		DatabaseURL:    databaseURL,
 		MigrationsDir:  DefaultMigrationsDir,
+		SchemaFile:     DefaultSchemaFile,
 	}
 }
 
@@ -34,8 +41,8 @@ func (db *DB) GetDriver() (Driver, error) {
 	return GetDriver(db.DatabaseURL.Scheme)
 }
 
-// Up creates the database (if necessary) and runs migrations
-func (db *DB) Up() error {
+// CreateAndMigrate creates the database (if necessary) and runs migrations
+func (db *DB) CreateAndMigrate() error {
 	drv, err := db.GetDriver()
 	if err != nil {
 		return err
@@ -75,10 +82,34 @@ func (db *DB) Drop() error {
 	return drv.DropDatabase(db.DatabaseURL)
 }
 
+// DumpSchema writes the current database schema to a file
+func (db *DB) DumpSchema() error {
+	drv, sqlDB, err := db.openDatabaseForMigration()
+	if err != nil {
+		return err
+	}
+	defer mustClose(sqlDB)
+
+	schema, err := drv.DumpSchema(db.DatabaseURL, sqlDB)
+	if err != nil {
+		return err
+	}
+
+	fmt.Printf("Writing: %s\n", db.SchemaFile)
+
+	// ensure schema directory exists
+	if err = ensureDir(filepath.Dir(db.SchemaFile)); err != nil {
+		return err
+	}
+
+	// write schema to file
+	return ioutil.WriteFile(db.SchemaFile, schema, 0644)
+}
+
 const migrationTemplate = "-- migrate:up\n\n\n-- migrate:down\n\n"
 
-// New creates a new migration file
-func (db *DB) New(name string) error {
+// NewMigration creates a new migration file
+func (db *DB) NewMigration(name string) error {
 	// new migration name
 	timestamp := time.Now().UTC().Format("20060102150405")
 	if name == "" {
@@ -87,8 +118,8 @@ func (db *DB) New(name string) error {
 	name = fmt.Sprintf("%s_%s.sql", timestamp, name)
 
 	// create migrations dir if missing
-	if err := os.MkdirAll(db.MigrationsDir, 0755); err != nil {
-		return fmt.Errorf("unable to create directory `%s`", db.MigrationsDir)
+	if err := ensureDir(db.MigrationsDir); err != nil {
+		return err
 	}
 
 	// check file does not already exist
@@ -203,6 +234,11 @@ func (db *DB) Migrate() error {
 
 	}
 
+	// automatically update schema file, silence errors
+	if db.AutoDumpSchema {
+		_ = db.DumpSchema()
+	}
+
 	return nil
 }
 
@@ -340,5 +376,10 @@ func (db *DB) Rollback() error {
 		return err
 	}
 
+	// automatically update schema file, silence errors
+	if db.AutoDumpSchema {
+		_ = db.DumpSchema()
+	}
+
 	return nil
 }

pkg/dbmate/db_test.go

@@ -1,6 +1,7 @@
 package dbmate
 
 import (
+	"io/ioutil"
 	"net/url"
 	"os"
 	"path/filepath"
@@ -23,7 +24,103 @@ func newTestDB(t *testing.T, u *url.URL) *DB {
 		require.Nil(t, err)
 	}
 
-	return NewDB(u)
+	db := New(u)
+	db.AutoDumpSchema = false
+
+	return db
+}
+
+func TestNew(t *testing.T) {
+	u := postgresTestURL(t)
+	db := New(u)
+	require.True(t, db.AutoDumpSchema)
+	require.Equal(t, u.String(), db.DatabaseURL.String())
+	require.Equal(t, "./db/migrations", db.MigrationsDir)
+	require.Equal(t, "./db/schema.sql", db.SchemaFile)
+}
+
+func TestDumpSchema(t *testing.T) {
+	u := postgresTestURL(t)
+	db := newTestDB(t, u)
+
+	// create custom schema file directory
+	dir, err := ioutil.TempDir("", "dbmate")
+	require.Nil(t, err)
+	defer func() {
+		err := os.RemoveAll(dir)
+		require.Nil(t, err)
+	}()
+
+	// create schema.sql in subdirectory to test creating directory
+	db.SchemaFile = filepath.Join(dir, "/schema/schema.sql")
+
+	// drop database
+	err = db.Drop()
+	require.Nil(t, err)
+
+	// create and migrate
+	err = db.CreateAndMigrate()
+	require.Nil(t, err)
+
+	// schema.sql should not exist
+	_, err = os.Stat(db.SchemaFile)
+	require.True(t, os.IsNotExist(err))
+
+	// dump schema
+	err = db.DumpSchema()
+	require.Nil(t, err)
+
+	// verify schema
+	schema, err := ioutil.ReadFile(db.SchemaFile)
+	require.Nil(t, err)
+	require.Contains(t, string(schema), "-- PostgreSQL database dump")
+}
+
+func TestAutoDumpSchema(t *testing.T) {
+	u := postgresTestURL(t)
+	db := newTestDB(t, u)
+	db.AutoDumpSchema = true
+
+	// create custom schema file directory
+	dir, err := ioutil.TempDir("", "dbmate")
+	require.Nil(t, err)
+	defer func() {
+		err := os.RemoveAll(dir)
+		require.Nil(t, err)
+	}()
+
+	// create schema.sql in subdirectory to test creating directory
+	db.SchemaFile = filepath.Join(dir, "/schema/schema.sql")
+
+	// drop database
+	err = db.Drop()
+	require.Nil(t, err)
+
+	// schema.sql should not exist
+	_, err = os.Stat(db.SchemaFile)
+	require.True(t, os.IsNotExist(err))
+
+	// create and migrate
+	err = db.CreateAndMigrate()
+	require.Nil(t, err)
+
+	// verify schema
+	schema, err := ioutil.ReadFile(db.SchemaFile)
+	require.Nil(t, err)
+	require.Contains(t, string(schema), "-- PostgreSQL database dump")
+
+	// remove schema
+	err = os.Remove(db.SchemaFile)
+	require.Nil(t, err)
+
+	// rollback
+	err = db.Rollback()
+	require.Nil(t, err)
+
+	// schema should be recreated
+	schema, err = ioutil.ReadFile(db.SchemaFile)
+	require.Nil(t, err)
+	require.Contains(t, string(schema), "-- PostgreSQL database dump")
 }
 
 func testURLs(t *testing.T) []*url.URL {
@@ -77,7 +174,7 @@ func testUpURL(t *testing.T, u *url.URL) {
 	require.Nil(t, err)
 
 	// create and migrate
-	err = db.Up()
+	err = db.CreateAndMigrate()
 	require.Nil(t, err)
 
 	// verify results

pkg/dbmate/driver.go

@@ -12,6 +12,7 @@ type Driver interface {
 	DatabaseExists(*url.URL) (bool, error)
 	CreateDatabase(*url.URL) error
 	DropDatabase(*url.URL) error
+	DumpSchema(*url.URL, *sql.DB) ([]byte, error)
 	CreateMigrationsTable(*sql.DB) error
 	SelectMigrations(*sql.DB, int) (map[string]bool, error)
 	InsertMigration(Transaction, string) error

pkg/dbmate/mysql.go

@@ -1,6 +1,7 @@
 package dbmate
 
 import (
+	"bytes"
 	"database/sql"
 	"fmt"
 	"net/url"
@@ -46,7 +47,7 @@ func (drv MySQLDriver) openRootDB(u *url.URL) (*sql.DB, error) {
 	return drv.Open(&rootURL)
 }
 
-func quoteIdentifier(str string) string {
+func mysqlQuoteIdentifier(str string) string {
 	str = strings.Replace(str, "`", "\\`", -1)
 
 	return fmt.Sprintf("`%s`", str)
@@ -64,7 +65,7 @@ func (drv MySQLDriver) CreateDatabase(u *url.URL) error {
 	defer mustClose(db)
 
 	_, err = db.Exec(fmt.Sprintf("create database %s",
-		quoteIdentifier(name)))
+		mysqlQuoteIdentifier(name)))
 
 	return err
 }
@@ -81,11 +82,77 @@ func (drv MySQLDriver) DropDatabase(u *url.URL) error {
 	defer mustClose(db)
 
 	_, err = db.Exec(fmt.Sprintf("drop database if exists %s",
-		quoteIdentifier(name)))
+		mysqlQuoteIdentifier(name)))
 
 	return err
 }
 
+func mysqldumpArgs(u *url.URL) []string {
+	// generate CLI arguments
+	args := []string{"--opt", "--routines", "--no-data",
+		"--skip-dump-date", "--skip-add-drop-table"}
+
+	if hostname := u.Hostname(); hostname != "" {
+		args = append(args, "--host="+hostname)
+	}
+	if port := u.Port(); port != "" {
+		args = append(args, "--port="+port)
+	}
+	if username := u.User.Username(); username != "" {
+		args = append(args, "--user="+username)
+	}
+	// mysql recommands against using environment variables to supply password
+	// https://dev.mysql.com/doc/refman/5.7/en/password-security-user.html
+	if password, set := u.User.Password(); set {
+		args = append(args, "--password="+password)
+	}
+
+	// add database name
+	args = append(args, strings.TrimLeft(u.Path, "/"))
+
+	return args
+}
+
+func mysqlSchemaMigrationsDump(db *sql.DB) ([]byte, error) {
+	// load applied migrations
+	migrations, err := queryColumn(db,
+		"select quote(version) from schema_migrations order by version asc")
+	if err != nil {
+		return nil, err
+	}
+
+	// build schema_migrations table data
+	var buf bytes.Buffer
+	buf.WriteString("\n--\n-- Dbmate schema migrations\n--\n\n" +
+		"LOCK TABLES `schema_migrations` WRITE;\n")
+
+	if len(migrations) > 0 {
+		buf.WriteString("INSERT INTO `schema_migrations` (version) VALUES\n  (" +
+			strings.Join(migrations, "),\n  (") +
+			");\n")
+	}
+
+	buf.WriteString("UNLOCK TABLES;\n")
+
+	return buf.Bytes(), nil
+}
+
+// DumpSchema returns the current database schema
+func (drv MySQLDriver) DumpSchema(u *url.URL, db *sql.DB) ([]byte, error) {
+	schema, err := runCommand("mysqldump", mysqldumpArgs(u)...)
+	if err != nil {
+		return nil, err
+	}
+
+	migrations, err := mysqlSchemaMigrationsDump(db)
+	if err != nil {
+		return nil, err
+	}
+
+	schema = append(schema, migrations...)
+	return trimLeadingSQLComments(schema)
+}
+
 // DatabaseExists determines whether the database exists
 func (drv MySQLDriver) DatabaseExists(u *url.URL) (bool, error) {
 	name := databaseName(u)
@@ -97,8 +164,8 @@ func (drv MySQLDriver) DatabaseExists(u *url.URL) (bool, error) {
 	defer mustClose(db)
 
 	exists := false
-	err = db.QueryRow(`select true from information_schema.schemata
-		where schema_name = ?`, name).Scan(&exists)
+	err = db.QueryRow("select true from information_schema.schemata "+
+		"where schema_name = ?", name).Scan(&exists)
 	if err == sql.ErrNoRows {
 		return false, nil
 	}
@@ -108,8 +175,8 @@ func (drv MySQLDriver) DatabaseExists(u *url.URL) (bool, error) {
 
 // CreateMigrationsTable creates the schema_migrations table
 func (drv MySQLDriver) CreateMigrationsTable(db *sql.DB) error {
-	_, err := db.Exec(`create table if not exists schema_migrations (
-		version varchar(255) primary key)`)
+	_, err := db.Exec("create table if not exists schema_migrations " +
+		"(version varchar(255) primary key)")
 
 	return err
 }

pkg/dbmate/mysql_test.go

@@ -90,6 +90,45 @@ func TestMySQLCreateDropDatabase(t *testing.T) {
 	}()
 }
 
+func TestMySQLDumpSchema(t *testing.T) {
+	drv := MySQLDriver{}
+	u := mySQLTestURL(t)
+
+	// prepare database
+	db := prepTestMySQLDB(t)
+	defer mustClose(db)
+	err := drv.CreateMigrationsTable(db)
+	require.Nil(t, err)
+
+	// insert migration
+	err = drv.InsertMigration(db, "abc1")
+	require.Nil(t, err)
+	err = drv.InsertMigration(db, "abc2")
+	require.Nil(t, err)
+
+	// DumpSchema should return schema
+	schema, err := drv.DumpSchema(u, db)
+	require.Nil(t, err)
+	require.Contains(t, string(schema), "CREATE TABLE `schema_migrations`")
+	require.Contains(t, string(schema), "\n-- Dump completed\n\n"+
+		"--\n"+
+		"-- Dbmate schema migrations\n"+
+		"--\n\n"+
+		"LOCK TABLES `schema_migrations` WRITE;\n"+
+		"INSERT INTO `schema_migrations` (version) VALUES\n"+
+		"  ('abc1'),\n"+
+		"  ('abc2');\n"+
+		"UNLOCK TABLES;\n")
+
+	// DumpSchema should return error if command fails
+	u.Path = "/fakedb"
+	schema, err = drv.DumpSchema(u, db)
+	require.Nil(t, schema)
+	require.NotNil(t, err)
+	require.Equal(t, "mysqldump: Got error: 1049: \"Unknown database 'fakedb'\" "+
+		"when selecting the database", err.Error())
+}
+
 func TestMySQLDatabaseExists(t *testing.T) {
 	drv := MySQLDriver{}
 	u := mySQLTestURL(t)

pkg/dbmate/postgres.go

@@ -1,9 +1,11 @@
 package dbmate
 
 import (
+	"bytes"
 	"database/sql"
 	"fmt"
 	"net/url"
+	"strings"
 
 	"github.com/lib/pq"
 )
@@ -59,6 +61,45 @@ func (drv PostgresDriver) DropDatabase(u *url.URL) error {
 	return err
 }
 
+func postgresSchemaMigrationsDump(db *sql.DB) ([]byte, error) {
+	// load applied migrations
+	migrations, err := queryColumn(db,
+		"select quote_literal(version) from schema_migrations order by version asc")
+	if err != nil {
+		return nil, err
+	}
+
+	// build schema_migrations table data
+	var buf bytes.Buffer
+	buf.WriteString("\n--\n-- Dbmate schema migrations\n--\n\n")
+
+	if len(migrations) > 0 {
+		buf.WriteString("INSERT INTO schema_migrations (version) VALUES\n    (" +
+			strings.Join(migrations, "),\n    (") +
+			");\n")
+	}
+
+	return buf.Bytes(), nil
+}
+
+// DumpSchema returns the current database schema
+func (drv PostgresDriver) DumpSchema(u *url.URL, db *sql.DB) ([]byte, error) {
+	// load schema
+	schema, err := runCommand("pg_dump", "--format=plain", "--encoding=UTF8",
+		"--schema-only", "--no-privileges", "--no-owner", u.String())
+	if err != nil {
+		return nil, err
+	}
+
+	migrations, err := postgresSchemaMigrationsDump(db)
+	if err != nil {
+		return nil, err
+	}
+
+	schema = append(schema, migrations...)
+	return trimLeadingSQLComments(schema)
+}
+
 // DatabaseExists determines whether the database exists
 func (drv PostgresDriver) DatabaseExists(u *url.URL) (bool, error) {
 	name := databaseName(u)
@@ -81,8 +122,8 @@ func (drv PostgresDriver) DatabaseExists(u *url.URL) (bool, error) {
 
 // CreateMigrationsTable creates the schema_migrations table
 func (drv PostgresDriver) CreateMigrationsTable(db *sql.DB) error {
-	_, err := db.Exec(`create table if not exists schema_migrations (
-		version varchar(255) primary key)`)
+	_, err := db.Exec("create table if not exists schema_migrations " +
+		"(version varchar(255) primary key)")
 
 	return err
 }

pkg/dbmate/postgres_test.go

@@ -72,6 +72,45 @@ func TestPostgresCreateDropDatabase(t *testing.T) {
 	}()
 }
 
+func TestPostgresDumpSchema(t *testing.T) {
+	drv := PostgresDriver{}
+	u := postgresTestURL(t)
+
+	// prepare database
+	db := prepTestPostgresDB(t)
+	defer mustClose(db)
+	err := drv.CreateMigrationsTable(db)
+	require.Nil(t, err)
+
+	// insert migration
+	err = drv.InsertMigration(db, "abc1")
+	require.Nil(t, err)
+	err = drv.InsertMigration(db, "abc2")
+	require.Nil(t, err)
+
+	// DumpSchema should return schema
+	schema, err := drv.DumpSchema(u, db)
+	require.Nil(t, err)
+	require.Contains(t, string(schema), "CREATE TABLE schema_migrations")
+	require.Contains(t, string(schema), "\n--\n"+
+		"-- PostgreSQL database dump complete\n"+
+		"--\n\n\n"+
+		"--\n"+
+		"-- Dbmate schema migrations\n"+
+		"--\n\n"+
+		"INSERT INTO schema_migrations (version) VALUES\n"+
+		"    ('abc1'),\n"+
+		"    ('abc2');\n")
+
+	// DumpSchema should return error if command fails
+	u.Path = "/fakedb"
+	schema, err = drv.DumpSchema(u, db)
+	require.Nil(t, schema)
+	require.NotNil(t, err)
+	require.Equal(t, "pg_dump: [archiver (db)] connection to database \"fakedb\" failed: "+
+		"FATAL:  database \"fakedb\" does not exist", err.Error())
+}
+
 func TestPostgresDatabaseExists(t *testing.T) {
 	drv := PostgresDriver{}
 	u := postgresTestURL(t)

pkg/dbmate/sqlite.go

@@ -1,11 +1,13 @@
 package dbmate
 
 import (
+	"bytes"
 	"database/sql"
 	"fmt"
 	"net/url"
 	"os"
 	"regexp"
+	"strings"
 
 	_ "github.com/mattn/go-sqlite3" // sqlite driver for database/sql
 )
@@ -56,6 +58,44 @@ func (drv SQLiteDriver) DropDatabase(u *url.URL) error {
 	return os.Remove(path)
 }
 
+func sqliteSchemaMigrationsDump(db *sql.DB) ([]byte, error) {
+	// load applied migrations
+	migrations, err := queryColumn(db,
+		"select quote(version) from schema_migrations order by version asc")
+	if err != nil {
+		return nil, err
+	}
+
+	// build schema_migrations table data
+	var buf bytes.Buffer
+	buf.WriteString("-- Dbmate schema migrations\n")
+
+	if len(migrations) > 0 {
+		buf.WriteString("INSERT INTO schema_migrations (version) VALUES\n  (" +
+			strings.Join(migrations, "),\n  (") +
+			");\n")
+	}
+
+	return buf.Bytes(), nil
+}
+
+// DumpSchema returns the current database schema
+func (drv SQLiteDriver) DumpSchema(u *url.URL, db *sql.DB) ([]byte, error) {
+	path := sqlitePath(u)
+	schema, err := runCommand("sqlite3", path, ".schema")
+	if err != nil {
+		return nil, err
+	}
+
+	migrations, err := sqliteSchemaMigrationsDump(db)
+	if err != nil {
+		return nil, err
+	}
+
+	schema = append(schema, migrations...)
+	return trimLeadingSQLComments(schema)
+}
+
 // DatabaseExists determines whether the database exists
 func (drv SQLiteDriver) DatabaseExists(u *url.URL) (bool, error) {
 	_, err := os.Stat(sqlitePath(u))
@@ -71,8 +111,8 @@ func (drv SQLiteDriver) DatabaseExists(u *url.URL) (bool, error) {
 
 // CreateMigrationsTable creates the schema_migrations table
 func (drv SQLiteDriver) CreateMigrationsTable(db *sql.DB) error {
-	_, err := db.Exec(`create table if not exists schema_migrations (
-		version varchar(255) primary key)`)
+	_, err := db.Exec("create table if not exists schema_migrations " +
+		"(version varchar(255) primary key)")
 
 	return err
 }

pkg/dbmate/sqlite_test.go

@@ -61,6 +61,40 @@ func TestSQLiteCreateDropDatabase(t *testing.T) {
 	require.Equal(t, true, os.IsNotExist(err))
 }
 
+func TestSQLiteDumpSchema(t *testing.T) {
+	drv := SQLiteDriver{}
+	u := sqliteTestURL(t)
+
+	// prepare database
+	db := prepTestSQLiteDB(t)
+	defer mustClose(db)
+	err := drv.CreateMigrationsTable(db)
+	require.Nil(t, err)
+
+	// insert migration
+	err = drv.InsertMigration(db, "abc1")
+	require.Nil(t, err)
+	err = drv.InsertMigration(db, "abc2")
+	require.Nil(t, err)
+
+	// DumpSchema should return schema
+	schema, err := drv.DumpSchema(u, db)
+	require.Nil(t, err)
+	require.Contains(t, string(schema), "CREATE TABLE schema_migrations")
+	require.Contains(t, string(schema), ");\n-- Dbmate schema migrations\n"+
+		"INSERT INTO schema_migrations (version) VALUES\n"+
+		"  ('abc1'),\n"+
+		"  ('abc2');\n")
+
+	// DumpSchema should return error if command fails
+	u.Path = "/."
+	schema, err = drv.DumpSchema(u, db)
+	require.Nil(t, schema)
+	require.NotNil(t, err)
+	require.Equal(t, "Error: unable to open database \".\": unable to open database file",
+		err.Error())
+}
+
 func TestSQLiteDatabaseExists(t *testing.T) {
 	drv := SQLiteDriver{}
 	u := sqliteTestURL(t)

pkg/dbmate/utils.go

@@ -1,8 +1,17 @@
 package dbmate
 
 import (
+	"bufio"
+	"bytes"
+	"database/sql"
+	"errors"
+	"fmt"
 	"io"
 	"net/url"
+	"os"
+	"os/exec"
+	"strings"
+	"unicode"
 )
 
 // databaseName returns the database name from a URL
@@ -15,8 +24,106 @@ func databaseName(u *url.URL) string {
 	return name
 }
 
+// mustClose ensures a stream is closed
 func mustClose(c io.Closer) {
 	if err := c.Close(); err != nil {
 		panic(err)
 	}
 }
+
+// ensureDir creates a directory if it does not already exist
+func ensureDir(dir string) error {
+	if err := os.MkdirAll(dir, 0755); err != nil {
+		return fmt.Errorf("unable to create directory `%s`", dir)
+	}
+
+	return nil
+}
+
+// runCommand runs a command and returns the stdout if successful
+func runCommand(name string, args ...string) ([]byte, error) {
+	var stdout, stderr bytes.Buffer
+	cmd := exec.Command(name, args...)
+	cmd.Stdout = &stdout
+	cmd.Stderr = &stderr
+
+	if err := cmd.Run(); err != nil {
+		// return stderr if available
+		if s := strings.TrimSpace(stderr.String()); s != "" {
+			return nil, errors.New(s)
+		}
+
+		// otherwise return error
+		return nil, err
+	}
+
+	// return stdout
+	return stdout.Bytes(), nil
+}
+
+// trimLeadingSQLComments removes sql comments and blank lines from the beginning of text
+// generally when performing sql dumps these contain host-specific information such as
+// client/server version numbers
+func trimLeadingSQLComments(data []byte) ([]byte, error) {
+	// create decent size buffer
+	out := bytes.NewBuffer(make([]byte, 0, len(data)))
+
+	// iterate over sql lines
+	preamble := true
+	scanner := bufio.NewScanner(bytes.NewReader(data))
+	for scanner.Scan() {
+		// we read bytes directly for premature performance optimization
+		line := scanner.Bytes()
+
+		if preamble && (len(line) == 0 || bytes.Equal(line[0:2], []byte("--"))) {
+			// header section, skip this line in output buffer
+			continue
+		}
+
+		// header section is over
+		preamble = false
+
+		// trim trailing whitespace
+		line = bytes.TrimRightFunc(line, unicode.IsSpace)
+
+		// copy bytes to output buffer
+		if _, err := out.Write(line); err != nil {
+			return nil, err
+		}
+		if _, err := out.WriteString("\n"); err != nil {
+			return nil, err
+		}
+	}
+	if err := scanner.Err(); err != nil {
+		return nil, err
+	}
+
+	return out.Bytes(), nil
+}
+
+// queryColumn runs a SQL statement and returns a slice of strings
+// it is assumed that the statement returns only one column
+// e.g. schema_migrations table
+func queryColumn(db *sql.DB, query string) ([]string, error) {
+	rows, err := db.Query(query)
+	if err != nil {
+		return nil, err
+	}
+	defer mustClose(rows)
+
+	// read into slice
+	var result []string
+	for rows.Next() {
+		var v string
+		if err := rows.Scan(&v); err != nil {
+			return nil, err
+		}
+
+		result = append(result, v)
+	}
+	if err = rows.Err(); err != nil {
+		return nil, err
+	}
+
+	return result, nil
+}

pkg/dbmate/utils_test.go

@@ -22,3 +22,14 @@ func TestDatabaseName_Empty(t *testing.T) {
 	name := databaseName(u)
 	require.Equal(t, "", name)
 }
+
+func TestTrimLeadingSQLComments(t *testing.T) {
+	in := "--\n" +
+		"-- foo\n\n" +
+		"-- bar\n\n" +
+		"real stuff\n" +
+		"-- end\n"
+	out, err := trimLeadingSQLComments([]byte(in))
+	require.Nil(t, err)
+	require.Equal(t, "real stuff\n-- end\n", string(out))
+}