Preserve default on column duplication when changing column type (#234)

When duplicating a column for backfilling to change a column's type,
ensure that any default on the column is preserved on the duplicated
column.

This fixes the issue where changing type would lose any `DEFAULT`
defined on the column

This is part of a larger class of issues where duplicated columns are
not faithfully preserving all properties of the original, tracked in
#227.

pkg/migrations/duplicate.go

@@ -42,14 +42,22 @@ func (d *Duplicator) WithType(t string) *Duplicator {
 func (d *Duplicator) Duplicate(ctx context.Context) error {
 	const (
 		cAlterTableSQL    = `ALTER TABLE %s ADD COLUMN %s %s`
+		cSetDefaultSQL    = `ALTER COLUMN %s SET DEFAULT %s`
 		cAddForeignKeySQL = `ADD CONSTRAINT %s FOREIGN KEY (%s) REFERENCES %s (%s)`
 	)
 
+	// Generate SQL to duplicate the column's name and type
 	sql := fmt.Sprintf(cAlterTableSQL,
 		pq.QuoteIdentifier(d.table.Name),
 		pq.QuoteIdentifier(d.asName),
 		d.withType)
 
+	// Generate SQL to duplicate the column's default value
+	if d.column.Default != nil {
+		sql += fmt.Sprintf(", "+cSetDefaultSQL, d.asName, *d.column.Default)
+	}
+
+	// Generate SQL to duplicate any foreign key constraints on the column
 	for _, fk := range d.table.ForeignKeys {
 		if slices.Contains(fk.Columns, d.column.Name) {
 			sql += fmt.Sprintf(", "+cAddForeignKeySQL,

pkg/migrations/op_change_type_test.go

@@ -222,6 +222,71 @@ func TestChangeColumnType(t *testing.T) {
 				ConstraintMustExist(t, db, "public", "employees", "fk_employee_department")
 			},
 		},
+		{
+			name: "changing column type preserves any defaults on the column",
+			migrations: []migrations.Migration{
+				{
+					Name: "01_add_table",
+					Operations: migrations.Operations{
+						&migrations.OpCreateTable{
+							Name: "users",
+							Columns: []migrations.Column{
+								{
+									Name: "id",
+									Type: "integer",
+									Pk:   true,
+								},
+								{
+									Name:     "username",
+									Type:     "text",
+									Default:  ptr("'alice'"),
+									Nullable: true,
+								},
+							},
+						},
+					},
+				},
+				{
+					Name: "02_change_type",
+					Operations: migrations.Operations{
+						&migrations.OpAlterColumn{
+							Table:  "users",
+							Column: "username",
+							Type:   "varchar(255)",
+							Up:     "username",
+							Down:   "username",
+						},
+					},
+				},
+			},
+			afterStart: func(t *testing.T, db *sql.DB) {
+				// A row can be inserted into the new version of the table.
+				MustInsert(t, db, "public", "02_change_type", "users", map[string]string{
+					"id": "1",
+				})
+
+				// The newly inserted row respects the default value of the column.
+				rows := MustSelect(t, db, "public", "02_change_type", "users")
+				assert.Equal(t, []map[string]any{
+					{"id": 1, "username": "alice"},
+				}, rows)
+			},
+			afterRollback: func(t *testing.T, db *sql.DB) {
+			},
+			afterComplete: func(t *testing.T, db *sql.DB) {
+				// A row can be inserted into the new version of the table.
+				MustInsert(t, db, "public", "02_change_type", "users", map[string]string{
+					"id": "2",
+				})
+
+				// The newly inserted row respects the default value of the column.
+				rows := MustSelect(t, db, "public", "02_change_type", "users")
+				assert.Equal(t, []map[string]any{
+					{"id": 1, "username": "alice"},
+					{"id": 2, "username": "alice"},
+				}, rows)
+			},
+		},
 	})
 }