fix: migrator for mysql 5.6 (#141)

migrator.go

@@ -224,6 +224,29 @@ func (m Migrator) RenameColumn(value interface{}, oldName, newName string) error
 	})
 }
 
+func (m Migrator) DropConstraint(value interface{}, name string) error {
+	if !m.Dialector.Config.DontSupportDropConstraint {
+		return m.Migrator.DropConstraint(value, name)
+	}
+
+	return m.RunWithValue(value, func(stmt *gorm.Statement) error {
+		constraint, table := m.GuessConstraintInterfaceAndTable(stmt, name)
+		if constraint != nil {
+			name = constraint.GetName()
+			switch constraint.(type) {
+			case *schema.Constraint:
+				return m.DB.Exec("ALTER TABLE ? DROP FOREIGN KEY ?", clause.Table{Name: table}, clause.Column{Name: name}).Error
+			case *schema.CheckConstraint:
+				return m.DB.Exec("ALTER TABLE ? DROP CHECK ?", clause.Table{Name: table}, clause.Column{Name: name}).Error
+			}
+		}
+		if m.HasIndex(value, name) {
+			return m.DB.Exec("ALTER TABLE ? DROP INDEX ?", clause.Table{Name: table}, clause.Column{Name: name}).Error
+		}
+		return nil
+	})
+}
+
 func (m Migrator) RenameIndex(value interface{}, oldName, newName string) error {
 	if !m.Dialector.DontSupportRenameIndex {
 		return m.RunWithValue(value, func(stmt *gorm.Statement) error {

mysql.go

@@ -41,6 +41,10 @@ type Config struct {
 	DontSupportForShareClause     bool
 	DontSupportNullAsDefaultValue bool
 	DontSupportRenameColumnUnique bool
+	// As of MySQL 8.0.19, ALTER TABLE permits more general (and SQL standard) syntax
+	// for dropping and altering existing constraints of any type.
+	// see https://dev.mysql.com/doc/refman/8.0/en/alter-table.html
+	DontSupportDropConstraint bool
 }
 
 type Dialector struct {
@@ -136,14 +140,17 @@ func (dialector Dialector) Initialize(db *gorm.DB) (err error) {
 			dialector.Config.DontSupportRenameIndex = true
 			dialector.Config.DontSupportRenameColumn = true
 			dialector.Config.DontSupportForShareClause = true
+			dialector.Config.DontSupportDropConstraint = true
 		} else if strings.HasPrefix(dialector.ServerVersion, "5.7.") {
 			dialector.Config.DontSupportRenameColumn = true
 			dialector.Config.DontSupportForShareClause = true
+			dialector.Config.DontSupportDropConstraint = true
 		} else if strings.HasPrefix(dialector.ServerVersion, "5.") {
 			dialector.Config.DisableDatetimePrecision = true
 			dialector.Config.DontSupportRenameIndex = true
 			dialector.Config.DontSupportRenameColumn = true
 			dialector.Config.DontSupportForShareClause = true
+			dialector.Config.DontSupportDropConstraint = true
 		}
 
 		if strings.Contains(dialector.ServerVersion, "TiDB") {