Test AUTO_INCREMENT only on postgres

Only the postgres dialect handles AUTO_INCREMENT on non-primary key.
So we skip the auto increment test for other dialects.

The mysql case is a little trickier because the simple presence of the
'AUTH_INCREMENT' tag produces a faulty 'CREATE TABLE' statement. Hence
we need to remove it when present.

create_test.go

@@ -58,6 +58,9 @@ func TestCreate(t *testing.T) {
 }
 
 func TestCreateWithAutoIncrement(t *testing.T) {
+	if dialect := os.Getenv("GORM_DIALECT"); dialect != "postgres" {
+		t.Skip("Skipping this because only postgres properly support auto_increment on a non-primary_key column")
+	}
 	user1 := User{}
 	user2 := User{}
 

dialect_mysql.go

@@ -30,6 +30,14 @@ func (mysql) Quote(key string) string {
 func (mysql) DataTypeOf(field *StructField) string {
 	var dataValue, sqlType, size, additionalType = ParseFieldStructForDialect(field)
 
+	// MySQL allows only one auto increment column per table, and it must
+	// be a KEY column.
+	if _, ok := field.TagSettings["AUTO_INCREMENT"]; ok {
+		if _, ok = field.TagSettings["INDEX"]; !ok && !field.IsPrimaryKey {
+			delete(field.TagSettings, "AUTO_INCREMENT")
+		}
+	}
+
 	if sqlType == "" {
 		switch dataValue.Kind() {
 		case reflect.Bool: