fix: quote string literals with single quotes in Explain

ddlmod.go

@@ -156,7 +156,9 @@ func parseDDL(strs ...string) (*ddl, error) {
 					}
 					if defaultMatches := defaultValueRegexp.FindStringSubmatch(matches[3]); len(defaultMatches) > 1 {
 						if strings.ToLower(defaultMatches[1]) != "null" {
-							columnType.DefaultValueValue = sql.NullString{String: strings.Trim(defaultMatches[1], `"`), Valid: true}
+							// single quotes are standard SQL string literals,
+							// double quotes come from tables created by older versions
+							columnType.DefaultValueValue = sql.NullString{String: trimQuote(defaultMatches[1]), Valid: true}
 						}
 					}
 
@@ -293,3 +295,14 @@ func (d *ddl) removeColumn(name string) bool {
 
 	return false
 }
+
+// trimQuote removes one pair of matching outer quotes from a default value
+// literal, so inner quotes of values like '"x"' survive.
+func trimQuote(s string) string {
+	if len(s) >= 2 {
+		if (s[0] == '\'' && s[len(s)-1] == '\'') || (s[0] == '"' && s[len(s)-1] == '"') {
+			return s[1 : len(s)-1]
+		}
+	}
+	return s
+}

sqlite.go

@@ -203,7 +203,8 @@ func (dialector Dialector) QuoteTo(writer clause.Writer, str string) {
 }
 
 func (dialector Dialector) Explain(sql string, vars ...interface{}) string {
-	return logger.ExplainSQL(sql, nil, `"`, vars...)
+	// single quotes: double quotes are identifier quoting in SQLite
+	return logger.ExplainSQL(sql, nil, `'`, vars...)
 }
 
 func (dialector Dialector) DataTypeOf(field *schema.Field) string {

sqlite_test.go

@@ -3,6 +3,7 @@ package sqlite
 import (
 	"database/sql"
 	"fmt"
+	"strings"
 	"testing"
 
 	"github.com/mattn/go-sqlite3"
@@ -130,3 +131,80 @@ func TestDialector(t *testing.T) {
 		})
 	}
 }
+
+func TestExplainQuotesStrings(t *testing.T) {
+	out := Dialector{}.Explain("SELECT * FROM t WHERE name = ?", "hello")
+	if !strings.Contains(out, "'hello'") {
+		t.Errorf("Explain must quote string literals with single quotes, got: %s", out)
+	}
+}
+
+type explainDefaultModel struct {
+	ID   int
+	Code string `gorm:"default:hello"`
+}
+
+func (explainDefaultModel) TableName() string { return "explain_defaults" }
+
+// GORM embeds string default values into the DDL via Dialector.Explain;
+// parseDDL must strip the single quotes (and double quotes from tables
+// created by older versions) so migrations stay idempotent.
+func TestDefaultValueRoundTrip(t *testing.T) {
+	db, err := gorm.Open(Open("file:explain_defaults?mode=memory&cache=shared"), &gorm.Config{})
+	if err != nil {
+		t.Fatalf("gorm.Open: %v", err)
+	}
+	t.Cleanup(func() {
+		if sqlDB, err := db.DB(); err == nil {
+			_ = sqlDB.Close()
+		}
+	})
+
+	if err := db.AutoMigrate(&explainDefaultModel{}); err != nil {
+		t.Fatal(err)
+	}
+	cols, err := db.Migrator().ColumnTypes(&explainDefaultModel{})
+	if err != nil {
+		t.Fatal(err)
+	}
+	for _, c := range cols {
+		if c.Name() == "code" {
+			if dv, ok := c.DefaultValue(); !ok || dv != "hello" {
+				t.Errorf("DefaultValue = (%q,%v), want (hello,true)", dv, ok)
+			}
+		}
+	}
+	// second AutoMigrate must not rebuild the table
+	var before string
+	if err := db.Raw("SELECT sql FROM sqlite_master WHERE type='table' AND name='explain_defaults'").Scan(&before).Error; err != nil {
+		t.Fatal(err)
+	}
+	if err := db.AutoMigrate(&explainDefaultModel{}); err != nil {
+		t.Fatal(err)
+	}
+	var after string
+	if err := db.Raw("SELECT sql FROM sqlite_master WHERE type='table' AND name='explain_defaults'").Scan(&after).Error; err != nil {
+		t.Fatal(err)
+	}
+	if before != after {
+		t.Errorf("DDL changed after second AutoMigrate:\n  before: %s\n  after:  %s", before, after)
+	}
+
+	// double quotes from tables created by older driver versions still parse
+	d, err := parseDDL("CREATE TABLE `legacy` (`code` text DEFAULT \"hi\")")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if dv, ok := d.columns[0].DefaultValue(); !ok || dv != "hi" {
+		t.Errorf("legacy DefaultValue = (%q,%v), want (hi,true)", dv, ok)
+	}
+
+	// only one outer quote pair is stripped; inner quotes survive
+	d, err = parseDDL("CREATE TABLE `q` (`code` text DEFAULT '\"x\"')")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if dv, ok := d.columns[0].DefaultValue(); !ok || dv != `"x"` {
+		t.Errorf(`quoted DefaultValue = (%q,%v), want ("x",true)`, dv, ok)
+	}
+}