visitField does not map custom types to underlying db types

[antonydenyer]

If you have a model with custom type and you create the table using NewCreateTable().Model() the type created in the db will be for type of the property rather than the underlying type that is produced.

e.g

type Hash struct {
	common.Hash
}

var _ driver.Valuer = (*Hash)(nil)

func (hash Hash) Value() (driver.Value, error) {
	return hash.Hash.ToHexString(), nil
}

var _ sql.Scanner = (*Hash)(nil)

func (hash *Hash) Scan(src interface{}) (err error) {
	switch src := src.(type) {
	case string:
		hash.Hash = common.HexStringToHash(src)
		return nil
	default:
		return fmt.Errorf("unsupported data type: %T", src)
	}
}

And when I do the following:

type MyModel struct {
  ID   uint64    `bun:",pk,autoincrement" json:"-"`
  Hash app.Hash  `json:"hash"`
}

_, err := db.NewCreateTable().
		Model((*MyModel)(nil)).
		Exec(ctx)

it will create the column with the type for a struct. This is fine when you're using sqllite as the underlying type is a string, but when you switch over to postgres the underlying type is jsonb. Which leads to a serialisation failure.

Currently (as best as I can tell) the only option is to use annotations of the model to specify the db type.

Would it be possible to extend the custom types to be able support something like:

func (hash Hash) SqlType() (driver.Type, error) {
	return sqltype.VarChar, nil
}

or perhaps to be able to register a specific mapping for migrations?

db.Dialect().AddSqlType(&MyModel{},  sqltype.VarChar)

Happy to take a stab at implementing. thoughts?