Function _get_difficult_cards

backend/src/stats/services.py:70–111

(
    session: Session,
    collection_id: uuid.UUID,
    min_attempts: int = 2,
    limit: int = 5,
)

def _get_difficult_cards(
    session: Session,
    collection_id: uuid.UUID,
    min_attempts: int = 2,
    limit: int = 5,
) -> list[CardBasicStats]:
    statement = (
        select(
            Card.id,
            Card.front,
            func.count(PracticeCard.id).label("total_attempts"),
            func.sum(case((PracticeCard.is_correct, 1), else_=0)).label(
                "correct_answers"
            ),
        )
        .join(PracticeCard, Card.id == PracticeCard.card_id)
        .join(PracticeSession, PracticeCard.session_id == PracticeSession.id)
        .where(
            Card.collection_id == collection_id,
            PracticeSession.is_completed,
            PracticeCard.is_practiced,
            PracticeCard.is_correct.is_not(None),
        )
        .group_by(Card.id, Card.front)
        .having(func.count(PracticeCard.id) >= min_attempts)
        .order_by(
            func.sum(case((PracticeCard.is_correct, 1), else_=0)).cast(Float)
            / func.count(PracticeCard.id)
        )
        .limit(limit)
    )

    results = session.exec(statement).all()
    return [
        CardBasicStats(
            id=card_id,
            front=front[:100],
            total_attempts=total_attempts,
            correct_answers=correct_answers,
        )
        for card_id, front, total_attempts, correct_answers in results
    ]


def get_collection_stats(