refactor: resolve the database dialect in one place

internal/database/analysis.go

@@ -11,7 +11,7 @@ import (
 	"gorm.io/gorm/clause"
 )
 
-var errUnsupportedAnalysisDialect = errors.New("unsupported database dialect for analysis merge")
+var errUnsupportedDialect = errors.New("unsupported database dialect for analysis merge")
 
 // MergeAnalysis atomically replaces the namespaces in patch while preserving
 // every namespace that is already stored but absent from patch. It writes
@@ -26,7 +26,12 @@ func MergeAnalysis(
 		patch = common.AnalysisData{}
 	}
 
-	expression, err := analysisMergeExpression(gormdb.Name(), patch)
+	dialect, err := DialectOf(gormdb)
+	if err != nil {
+		return nil, err
+	}
+
+	expression, err := analysisMergeExpression(dialect, patch)
 	if err != nil {
 		return nil, err
 	}
@@ -57,7 +62,7 @@ func MergeAnalysis(
 	return merged, nil
 }
 
-func analysisMergeExpression(dialect string, patch common.AnalysisData) (clause.Expr, error) {
+func analysisMergeExpression(dialect Dialect, patch common.AnalysisData) (clause.Expr, error) {
 	patchJSON, err := json.Marshal(patch)
 	if err != nil {
 		return clause.Expr{}, fmt.Errorf("marshal analysis patch: %w", err)
@@ -94,6 +99,6 @@ func analysisMergeExpression(dialect string, patch common.AnalysisData) (clause.
 			string(patchJSON),
 		), nil
 	default:
-		return clause.Expr{}, fmt.Errorf("%w: %s", errUnsupportedAnalysisDialect, dialect)
+		return clause.Expr{}, fmt.Errorf("%w: %q", errUnsupportedDialect, dialect)
 	}
 }

internal/database/analysis_test.go

@@ -220,7 +220,7 @@ func openAnalysisTestDatabase(t *testing.T) *gorm.DB {
 	if dsn != "" {
 		var err error
 
-		dialect, err = Dialect(dsn)
+		dialect, err = DialectFromDSN(dsn)
 		require.NoError(t, err)
 	}
 

internal/database/database.go

@@ -14,19 +14,23 @@ import (
 	"gorm.io/gorm/schema"
 )
 
+type Dialect string
+
 const (
-	SQLite   = "sqlite"
-	Postgres = "postgres"
+	SQLite   Dialect = "sqlite"
+	Postgres Dialect = "postgres"
 )
 
 var ErrUnknownDialect = errors.New(
 	`unrecognized database DSN: expected a "file:" SQLite DSN, a "postgres://" URL or a key=value connection string`,
 )
 
-// Dialect reports which database a DSN points at. Everything that opens
-// a connection goes through here, so the application and the test
+var errUnknownGormDialect = errors.New("unrecognized gorm driver name")
+
+// DialectFromDSN reports which database a DSN points at. Everything that
+// opens a connection goes through here, so the application and the test
 // harness can't disagree and end up talking to two different databases.
-func Dialect(connection string) (string, error) {
+func DialectFromDSN(connection string) (Dialect, error) {
 	switch {
 	case strings.HasPrefix(connection, "file:"):
 		return SQLite, nil
@@ -39,8 +43,23 @@ func Dialect(connection string) (string, error) {
 	return "", fmt.Errorf("%w: %q", ErrUnknownDialect, connection)
 }
 
+// DialectOf reports the Dialect an already open *gorm.DB is talking to, read
+// from the driver name gorm reports for it. It is the one place that maps a
+// gorm driver name back to a Dialect, so a request handler and NewDatabase
+// read the same fact instead of each guessing from the connection.
+func DialectOf(gormdb *gorm.DB) (Dialect, error) {
+	switch name := gormdb.Name(); name {
+	case string(SQLite):
+		return SQLite, nil
+	case string(Postgres):
+		return Postgres, nil
+	default:
+		return "", fmt.Errorf("%w: %q", errUnknownGormDialect, name)
+	}
+}
+
 func NewDatabase(connection string) (*gorm.DB, error) {
-	dialect, err := Dialect(connection)
+	dialect, err := DialectFromDSN(connection)
 	if err != nil {
 		return nil, err
 	}
@@ -52,7 +71,7 @@ func NewDatabase(connection string) (*gorm.DB, error) {
 		log.Println("using SQLite database")
 
 		database, err = gorm.Open(sqlite.Open(connection), &gorm.Config{})
-	default:
+	case Postgres:
 		log.Println("using Postgres database")
 
 		database, err = gorm.Open(postgres.Open(connection), &gorm.Config{

internal/database/database_test.go

@@ -26,7 +26,7 @@ func testConnection(t *testing.T) string {
 		return sqliteTestConnection(t)
 	}
 
-	dialect, err := Dialect(connection)
+	dialect, err := DialectFromDSN(connection)
 	require.NoError(t, err)
 
 	if dialect == SQLite {

main_test.go

@@ -72,13 +72,13 @@ func init() {
 
 	dsn := os.Getenv("DATABASE_DSN")
 
-	dialect, err := database.Dialect(dsn)
+	dialect, err := database.DialectFromDSN(dsn)
 	if err != nil {
 		log.Fatal(err)
 	}
 
 	// database/sql registers the SQLite driver under a name of its own.
-	dbDriver = dialect
+	dbDriver = string(dialect)
 	if dialect == database.SQLite {
 		dbDriver = "sqlite3"
 	}