fix: ignore add constraint on unique index (#157)

Author: a631807682

migrator.go

@@ -336,8 +336,11 @@ func (m Migrator) AlterColumn(value interface{}, field string) error {
 
 			if uniq, _ := fieldColumnType.Unique(); !uniq && field.Unique {
 				idxName := clause.Column{Name: m.DB.Config.NamingStrategy.IndexName(stmt.Table, field.DBName)}
-				if err := m.DB.Exec("ALTER TABLE ? ADD CONSTRAINT ? UNIQUE(?)", m.CurrentTable(stmt), idxName, clause.Column{Name: field.DBName}).Error; err != nil {
-					return err
+				// Not a unique constraint but a unique index
+				if !m.HasIndex(stmt.Table, idxName.Name) {
+					if err := m.DB.Exec("ALTER TABLE ? ADD CONSTRAINT ? UNIQUE(?)", m.CurrentTable(stmt), idxName, clause.Column{Name: field.DBName}).Error; err != nil {
+						return err
+					}
 				}
 			}