Override SQL-to-Go types

Choose the scope of a Go type override: one query output, one schema column, or every occurrence of an SQL type. This example represents money as an application type without changing the integer stored in SQLite.

Choose where the override applies

To use menu.Pence for a column’s model field, direct results and parameters, put the qualified-column directive in its migration:

-- sgl.column(menu_items.price_pence, go_type: "seagull.sh/seagull/examples/sqlite/009_overrides/internal/menu.Pence")
CREATE TABLE menu_items (
    id          INTEGER PRIMARY KEY,
    name        TEXT    NOT NULL,
    section     TEXT    NOT NULL,
    status      TEXT    NOT NULL CHECK (status IN ('available', 'sold_out')),
    price_pence INTEGER NOT NULL,
    nutrition   TEXT    NOT NULL
);

For a derived result used only by one query, put the override in that query’s header:

-- sgl.query(CheapestPerSection, many)
-- sgl.column(cheapest, go_type: "seagull.sh/seagull/examples/sqlite/009_overrides/internal/menu.Pence", nullable: false)
SELECT section, MIN(price_pence) AS cheapest
FROM menu_items
GROUP BY section
ORDER BY section;

Replace the example’s import path with your own type’s package. Assert nullable: false only when every returned group has a non-null minimum. For a project-wide override, use go.types.overrides; the example’s configuration shows a column override.

Generate and read the application type

Run sgl generate and inspect the changed field before adapting the caller. The money type supplies the application’s formatting methods; the caller uses the generated result directly:

	cheapest, err := queries.CheapestPerSection(ctx)
	if err != nil {
		return fmt.Errorf("finding the cheapest items: %w", err)
	}
	for _, section := range cheapest {
		fmt.Fprintf(output, "cheapest %-6s from %s\n", section.Section, section.Cheapest)
	}
	return nil

Keep the driver contract in view: a defined integer type can use the normal integer path, while a different representation may need Scan and Value. Test both binding and reading, including NULL if the field permits it.

Check binding and scanning

From the Seagull repository root, run the override example:

cd examples/sqlite/009_overrides
go test .

The example test provides its local inputs. See its caller for the exact Go API and its test for assertions and failure handling.

The complete example produces:

Cod and chips       £9.50  1050 kcal
Haddock and chips  sold out
Chips               £3.50   560 kcal
Mushy peas          £1.75   120 kcal
cheapest fish   from £9.50
cheapest sides  from £1.75

Keep overrides within their scope

A query-level directive overrides migration and configured mappings for that output. Direct passthrough columns preserve a catalogue override; computations, CTEs, derived tables and casts to another SQL type can lose it. A cast to the column’s own type keeps it; one with other modifiers, such as code::varchar(4) of a varchar(8) column, is a cast to another type. Configuration expects value types without a leading pointer; nullability supplies the pointer when needed.

See the type mapping reference for the complete contract. The example browser exposes other engine variants; use their actual SQL rather than changing only the dialect name.