Dev Meeting 002: Intro to DDD

# Value Object — defined by its attributes, no identity
class ISRC:
    def __init__(self, code: str):
        if not self._validate(code):
            raise ValueError(f"Invalid ISRC: {code}")
        self.code = code

    @staticmethod
    def _validate(code: str) -> bool:
        return len(code) == 12 and code[:2].isalpha()

    def __eq__(self, other):
        return isinstance(other, ISRC) and self.code == other.code


# Entity — has identity (id), mutable state
class Track:
    def __init__(self, track_id: str, title: str, isrc: ISRC, duration_seconds: int):
        self.track_id = track_id
        self.title = title
        self.isrc = isrc
        self.duration_seconds = duration_seconds


# Aggregate Root — controls access to child entities
class Release:
    def __init__(self, release_id: str, title: str, artist: str):
        self.release_id = release_id
        self.title = title
        self.artist = artist
        self._tracks: list[Track] = []

    def add_track(self, track: Track):
        if any(t.isrc == track.isrc for t in self._tracks):
            raise ValueError(f"Track with ISRC {track.isrc.code} already exists")
        self._tracks.append(track)

    @property
    def total_duration(self) -> int:
        return sum(t.duration_seconds for t in self._tracks)