fix(Migrator): don't panic in AlterColumn/DropColumn when value is a table-name string

migrator.go

@@ -78,6 +78,9 @@ func (m Migrator) HasColumn(value interface{}, name string) bool {
 func (m Migrator) AlterColumn(value interface{}, name string) error {
 	return m.RunWithoutForeignKey(func() error {
 		return m.recreateTable(value, nil, func(ddl *ddl, stmt *gorm.Statement) (*ddl, []interface{}, error) {
+			if stmt.Schema == nil {
+				return nil, nil, fmt.Errorf("failed to alter field with name %v: model with schema is required", name)
+			}
 			if field := stmt.Schema.LookUpField(name); field != nil {
 				var sqlArgs []interface{}
 				for i, f := range ddl.fields {
@@ -157,8 +160,10 @@ func (m Migrator) ColumnTypes(value interface{}) ([]gorm.ColumnType, error) {
 func (m Migrator) DropColumn(value interface{}, name string) error {
 	return m.RunWithoutForeignKey(func() error {
 		return m.recreateTable(value, nil, func(ddl *ddl, stmt *gorm.Statement) (*ddl, []interface{}, error) {
-			if field := stmt.Schema.LookUpField(name); field != nil {
-				name = field.DBName
+			if stmt.Schema != nil {
+				if field := stmt.Schema.LookUpField(name); field != nil {
+					name = field.DBName
+				}
 			}
 
 			ddl.removeColumn(name)

migrator_test.go

@@ -0,0 +1,40 @@
+package sqlite
+
+import (
+	"testing"
+
+	"gorm.io/gorm"
+)
+
+// DropColumn and AlterColumn must accept a table-name string like the base
+// GORM migrator and the other dialects do; stmt.Schema is nil in that case
+// and used to cause a nil pointer dereference.
+func TestMigratorStringTableName(t *testing.T) {
+	db, err := gorm.Open(Open("file:migrator_string_value?mode=memory&cache=shared"), &gorm.Config{})
+	if err != nil {
+		t.Fatalf("gorm.Open: %v", err)
+	}
+	// close the pool so the shared in-memory database is torn down and
+	// repeated runs (go test -count=N) start from a clean state
+	t.Cleanup(func() {
+		if sqlDB, err := db.DB(); err == nil {
+			_ = sqlDB.Close()
+		}
+	})
+	if err := db.Exec("CREATE TABLE `string_value_table` (`id` integer, `b` text)").Error; err != nil {
+		t.Fatal(err)
+	}
+
+	if err := db.Migrator().DropColumn("string_value_table", "b"); err != nil {
+		t.Errorf("DropColumn with a table-name string: %v", err)
+	}
+	if db.Migrator().HasColumn("string_value_table", "b") {
+		t.Error("column b still present after DropColumn")
+	}
+
+	// AlterColumn needs the model schema to build the new column type; a
+	// table-name string must produce a regular error, not a panic.
+	if err := db.Migrator().AlterColumn("string_value_table", "id"); err == nil {
+		t.Error("AlterColumn with a table-name string: expected an error, got nil")
+	}
+}