admin: songs: Add a filter on reportnote count

This filter allows the admin to list only the songs that have a
report note that needs to be resolved.

marietje/songs/admin.py

@@ -1,4 +1,5 @@
 from django.contrib import admin
+from django.db.models import Count
 
 from .models import ReportNote, Song
 
@@ -7,15 +8,37 @@ class ReportNoteInline(admin.StackedInline):
     model = ReportNote
     extra = 0
 
+class SongHasReportNoteFilter(admin.SimpleListFilter):
+    title = 'report notes'
+    parameter_name = 'reportnotes'
+
+    def lookups(self, request, model_admin):
+        return (
+            ('yes', 'yes'),
+            ('no', 'no'),
+        )
+
+    def queryset(self, request, queryset):
+        queryset = queryset.annotate(num_reports=Count('reportnote'))
+        if self.value() == 'yes':
+            return queryset.exclude(num_reports=0)
+        if self.value() == 'no':
+            return queryset.filter(num_reports=0)
+        return queryset
+
+
 @admin.register(Song)
 class SongAdmin(admin.ModelAdmin):
     list_display = ('artist', 'title', 'user_name', 'reports')
+    list_display_links = ('artist', 'title')
+    list_filter = (SongHasReportNoteFilter,)
     search_fields = ('artist', 'title', 'user__name')
     inlines = [ReportNoteInline]
 
     @staticmethod
     def reports(song):
-        return ReportNote.objects.filter(song=song).count()
+        # num_reports is annotated by SongHasReportNoteFilter
+        return song.num_reports
 
     @staticmethod
     def user_name(song):