SQLAlchemy: show only latest result if a join returns multiple results

Question

I'm trying to create a Flask app that shows the latest score from individual players. So a player can have multiple scores, but on the leaderboard I only want to show her most recent score.

My models.py:

class Player(db.Model):
    __tablename__ = 'player'
    id = db.Column(db.Integer, primary_key=True)
    firstname = db.Column(db.String, nullable=False)
    score = db.relationship('Score', backref='player', lazy='dynamic')

    def __init__(self, firstname):
        self.firstname = firstname

    def __repr__(self):
        return '<id {}>'.format(self.id)


class Score(db.Model):
    __tablename__ = 'score'
    id = db.Column(db.Integer, primary_key=True)
    timestamp = db.Column(db.DateTime, nullable=False)
    score = db.Column(db.String, nullable=True)
    player_id = db.Column(db.Integer, db.ForeignKey('player.id'))

    def __init__(self, score, player_id):
        self.timestamp = datetime.now()
        self.score = score
        self.player_id = player_id

    def __repr__(self):
        return '<id {}>'.format(self.id)

In my app.py I have the following:

@app.route('/', methods=['GET', 'POST'])
@login_required
def home():
    """Render homepage"""

    players_with_score = db.session.query(
        Player, Score).join(Score)

    return render_template('home.html', players_with_score=players_with_score)

And in my (simplified) template home.html:

<tbody>
  {% for players_with_score in players_with_score %}
  <tr>
    <td>{{ players_with_score.Player.firstname }}</td>
    <td>{{ players_with_score.Score.score }}</td>
    <td>{{ players_with_score.Score.timestamp }}</td>
  </tr>
  {% endfor %}
</tbody>

This results in a proper table with all the scores, but now if a player has multiple scores, it will show that player also multiple times.

What do I need to do in the query to ensure only the latest score per player is shown? I tried adding .(Score.timestamp == db.session.query(func.max(Score.timestamp))) but that made that the only result was the latest score, not the latest score per player.

Answer 1

You can use the DISTINCT ON ... ORDER BY "idiom" in Postgresql to get the greatest-n-per-group, where n equals 1, given that your table isn't huge:

players_with_score = db.session.query(Player, Score).\
    join(Score).\
    distinct(Player.id).\
    order_by(Player.id, Score.timestamp.desc())

Since Score.timestamp is not nullable, you do not need to worry about descending order and NULLs.