fix:Issue migrating field with CURRENT_TIMESTAMP (#5906)

Co-authored-by: ningfei <accelerator314@outlook.com>

migrator/migrator.go

@@ -470,17 +470,19 @@ func (m Migrator) MigrateColumn(value interface{}, field *schema.Field, columnTy
 
 	// check default value
 	if !field.PrimaryKey {
+		currentDefaultNotNull := field.HasDefaultValue && !strings.EqualFold(field.DefaultValue, "NULL")
 		dv, dvNotNull := columnType.DefaultValue()
-		if dvNotNull && field.DefaultValueInterface == nil {
+		if dvNotNull && !currentDefaultNotNull {
 			// defalut value -> null
 			alterColumn = true
-		} else if !dvNotNull && field.DefaultValueInterface != nil {
+		} else if !dvNotNull && currentDefaultNotNull {
 			// null -> default value
 			alterColumn = true
-		} else if dv != field.DefaultValue {
+		} else if (field.GORMDataType != schema.Time && dv != field.DefaultValue) ||
+			(field.GORMDataType == schema.Time && !strings.EqualFold(strings.TrimSuffix(dv, "()"), strings.TrimSuffix(field.DefaultValue, "()"))) {
 			// default value not equal
 			// not both null
-			if !(field.DefaultValueInterface == nil && !dvNotNull) {
+			if currentDefaultNotNull || dvNotNull {
 				alterColumn = true
 			}
 		}

tests/migrate_test.go

@@ -757,6 +757,32 @@ func TestPrimarykeyID(t *testing.T) {
 	}
 }
 
+func TestCurrentTimestamp(t *testing.T) {
+	if DB.Dialector.Name() != "mysql" {
+		return
+	}
+	type CurrentTimestampTest struct {
+		ID     string     `gorm:"primary_key"`
+		TimeAt *time.Time `gorm:"type:datetime;not null;default:CURRENT_TIMESTAMP;unique"`
+	}
+	var err error
+	err = DB.Migrator().DropTable(&CurrentTimestampTest{})
+	if err != nil {
+		t.Errorf("DropTable err:%v", err)
+	}
+	err = DB.AutoMigrate(&CurrentTimestampTest{})
+	if err != nil {
+		t.Fatalf("AutoMigrate err:%v", err)
+	}
+
+	err = DB.AutoMigrate(&CurrentTimestampTest{})
+	if err != nil {
+		t.Fatalf("AutoMigrate err:%v", err)
+	}
+	AssertEqual(t, true, DB.Migrator().HasIndex(&CurrentTimestampTest{}, "time_at"))
+	AssertEqual(t, false, DB.Migrator().HasIndex(&CurrentTimestampTest{}, "time_at_2"))
+}
+
 func TestUniqueColumn(t *testing.T) {
 	if DB.Dialector.Name() != "mysql" {
 		return