def location_candidates(profile, preferred=None):
    candidates = []
    seen = set()

    def add_location_id(source):
        if isinstance(source, dict):
            value = source.get('id') or source.get('location_id')
        else:
            value = getattr(source, 'id', None)

        if value is None or isinstance(value, bool):
            return

        try:
            location_id = int(value)
        except (TypeError, ValueError):
            return

        if location_id not in seen:
            seen.add(location_id)
            candidates.append(location_id)

    if preferred:
        try:
            add_location_id({'id': int(preferred)})
        except (TypeError, ValueError):
            pass

    sources = []
    try:
        if profile and profile.location:
            sources.append(profile.location)
    except Exception:
        pass
    try:
        if profile and profile.connection and profile.connection.location:
            sources.append(profile.connection.location)
    except Exception:
        pass

    for source in sources:
        add_location_id(source)

    return candidates