docs: add a TODO; also add IsSqlitePrimaryKeyError and change ForeignKey interface to '~int'

doc/TODO.txt

@@ -8,3 +8,6 @@ TODO: auto-timestamps
 TODO: modified-timestamps
 - set updated_at and created_at in SaveXYZ
 - soft delete option
+
+TODO: generator-foreign-keys
+- add auto-foreign-key checking blocks to SaveXyz

pkg/db/errors.go

@@ -15,7 +15,7 @@ var (
 )
 
 type ForeignKey interface {
-	~uint64 | ~string
+	~int
 }
 
 type ForeignKeyError[T ForeignKey] struct {
@@ -65,6 +65,18 @@ func IsSqliteUniqError(err error) bool {
 	return errors.Is(sqliteErr.ExtendedCode, sqlite3.ErrConstraintUnique)
 }
 
+// IsSqlitePrimaryKeyError checks whether an error is a SQLite `primary key` constraint violation.
+//
+// TODO: it's kind of annoying that SQLite returns this instead of whether the violation was
+// .     uniqueness or null-ness.
+func IsSqlitePrimaryKeyError(err error) bool {
+	var sqliteErr sqlite3.Error
+	if !errors.As(err, &sqliteErr) {
+		return false
+	}
+	return errors.Is(sqliteErr.ExtendedCode, sqlite3.ErrConstraintPrimaryKey)
+}
+
 // IsSqliteNotNullError checks whether an error is a SQLite not null constraint violation.
 func IsSqliteNotNullError(err error) bool {
 	var sqliteErr sqlite3.Error