Welcome to the Treehouse Community

Want to collaborate on code errors? Have bugs you need feedback on? Looking for an extra set of eyes on your latest project? Get support with fellow developers, designers, and programmers of all backgrounds and skill levels here with the Treehouse Community! While you're at it, check out some resources Treehouse students have shared here.

Looking to learn something new?

Treehouse offers a seven day free trial for new students. Get access to thousands of hours of content and join thousands of Treehouse students and alumni in the community today.

Start your free trial

Python Django REST Framework Security and Customization Add Custom Field

Lewis Cowles
Lewis Cowles
74,902 Points
Posted by Lewis Cowles
Lewis Cowles
74,902 Points

Syntax for getting average of linked objects?

So I'm stuck on this, I'm sure it will come to me (if it does I'm answering my own question), but I'd like to know if anyone has links to Django or DRF docs for this?

scorekeeper/serializers.py 
from django.db.models import Avg

from rest_framework import serializers

from . import models


class GameSerializer(serializers.ModelSerializer):
    average_score = serializers.SerializerMethodField()
    class Meta:
        fields = ('id', 'name', 'average_score',)
        model = models.Game

    def get_average_score(self, obj):
        average = Game.scores.all().aggregate(Avg('score')).get('score__avg')

        if average is None:
            return 0

        return round(average*2)/2

class PlayerSerializer(serializers.ModelSerializer):
    class Meta:
        fields = ('id', 'name', 'age', 'parent_approval')
        model = models.Player

    def validate(self, data):
        if data['age'] < 13 and not data['parent_approval']:
            raise serializers.ValidationError(
                "Children under 13 require parental approval"
            )
        return data


class ScoreSerializer(serializers.ModelSerializer):
    game = serializers.HyperlinkedRelatedField(
        read_only=True, view_name='apiv2:game-detail')
    player = serializers.HyperlinkedRelatedField(
        read_only=True, view_name='apiv2:player-detail')

    class Meta:
        fields = ('id', 'game', 'player', 'score')
        model = models.Score

1 Answer

Lewis Cowles
Lewis Cowles
74,902 Points
Lewis Cowles
Lewis Cowles
74,902 Points

So this was pretty simple... It needs to be a specific instance of Game (obj). Also there was no need to make average an integer or do any rounding, so that helped

Final Class

class GameSerializer(serializers.ModelSerializer):
    average_score = serializers.SerializerMethodField()
    class Meta:
        fields = ('id', 'name', 'average_score')
        model = models.Game
    def get_average_score(self, obj):
        average = obj.scores.aggregate(Avg('score')).get('score__avg')
        if average is None:
            return 0
        return average