API: make frontend_url absolute

ephios/api/views/events.py

@@ -1,4 +1,7 @@
+from urllib.parse import urljoin
+
 import django_filters
+from django.conf import settings
 from django.db.models import Max, Min, Prefetch
 from oauth2_provider.contrib.rest_framework import IsAuthenticatedOrTokenHasScope
 from rest_framework import filters, serializers, viewsets
@@ -47,7 +50,10 @@ class EventSerializer(serializers.ModelSerializer):
     end_time = serializers.DateTimeField(source="get_end_time")
     signup_stats = SignupStatsSerializer(source="get_signup_stats")
     shifts = ShiftSerializer(many=True)
-    frontend_url = serializers.URLField(source="get_absolute_url")
+    frontend_url = serializers.SerializerMethodField()
+
+    def get_frontend_url(self, obj):
+        return urljoin(settings.GET_SITE_URL(), obj.get_absolute_url())
 
     class Meta:
         model = Event