Bug: CreateTableQuery fails to set VARCHAR length (MSSQL and MySQL)

[bevzzz]

I've encountered this odd behavior with CreateTableQuery.Varchar(n):

// Regular query
db.NewCreateTable().Model(new(Model))
// Output:
// pg:			CREATE TABLE "models" ("id" VARCHAR)
// pgx:			CREATE TABLE "models" ("id" VARCHAR)
// sqlite:		CREATE TABLE "models" ("id" VARCHAR)
// mysql5:		CREATE TABLE `models` (`id` VARCHAR(255))
// mysql8:		CREATE TABLE `models` (`id` VARCHAR(255))
// mariadb:		CREATE TABLE `models` (`id` VARCHAR(255))
// mssql2019: 	        CREATE TABLE "models" ("id" VARCHAR(255))

// Specify VARCHAR length
db.NewCreateTable().Model(new(Model)).Varchar(10)
// Output:
// pg:        CREATE TABLE "models" ("id" varchar(10))
// pgx:       CREATE TABLE "models" ("id" varchar(10))
// sqlite:    CREATE TABLE "models" ("id" varchar(10))
// mysql5:    CREATE TABLE `models` (`id` VARCHAR(255))
// mysql8:    CREATE TABLE `models` (`id` VARCHAR(255))
// mariadb:   CREATE TABLE `models` (`id` VARCHAR(255))
// mssql2019: CREATE TABLE "models" ("id" VARCHAR(255))

I've also noticed there is no unit tests that check .Varchar() method, so I guess it could be a bug.

Problem
Specifying VARCHAR length is mandatory in MySQL and MSSQL, so the default 255 is hardcoded in their DiscoveredSQLType:

bun/dialect/mssqldialect/dialect.go

Line 133 in 733affd

return field.DiscoveredSQLType + "(255)"

bun/dialect/mysqldialect/dialect.go

Line 178 in 733affd

return field.DiscoveredSQLType + "(255)"

As a result, this function in query_table_create.go fails to recognize VARCHAR column in those dialects :

Click for a commented version

func (q *CreateTableQuery) appendSQLType(b []byte, field *schema.Field) []byte {
        // Most of the time these two will be identical, but for the cases where DiscoveredSQLType is dialect-specific,
        // e.g. pgdialect would change sqltype.SmallInt to pgTypeSmallSerial for columns that have `bun:",autoincrement"`
	if field.CreateTableSQLType != field.DiscoveredSQLType {
		return append(b, field.CreateTableSQLType...)
	}

        // For all common SQL types except VARCHAR, both UserDefinedSQLType and DiscoveredSQLType specify the correct type,
       // and we needn't modify it. For VARCHAR columns, we will stop to check if a valid length has been set in .Varchar(int).
	if q.varchar > 0 &&
		field.CreateTableSQLType == sqltype.VarChar {
		b = append(b, "varchar("...)
		b = strconv.AppendInt(b, int64(q.varchar), 10)
		b = append(b, ")"...)
		return b
	}

	return append(b, field.CreateTableSQLType...)
}

Solution
We could have each dialect provide a method like d.DefaultVarcharLen() and then use that value if no other valid length is specified.
Dialects for which specifying the length is not mandatory (PG, SQLite) could simply return a 0, which would be ignored.

I can do a PR with a fix, if that's indeed a bug and the solution is OK.

[vmihailenco]

I guess this is fixed by #738