switch media.can_download flag to a db value

app/sync/admin.py

@@ -15,6 +15,6 @@ class SourceAdmin(admin.ModelAdmin):
 class MediaAdmin(admin.ModelAdmin):
 
     ordering = ('-created',)
-    list_display = ('key', 'source')
+    list_display = ('key', 'source', 'can_download', 'downloaded')
     readonly_fields = ('uuid', 'created')
     search_fields = ('uuid', 'source__key', 'key')

app/sync/migrations/0018_media_can_download.py

@@ -0,0 +1,18 @@
+# Generated by Django 3.1.4 on 2020-12-09 08:40
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('sync', '0017_auto_20201208_0521'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='media',
+            name='can_download',
+            field=models.BooleanField(db_index=True, default=False, help_text='Media has a matching format and can be downloaded', verbose_name='can download'),
+        ),
+    ]

app/sync/models.py

@@ -439,6 +439,12 @@ class Media(models.Model):
         null=True,
         help_text=_('JSON encoded metadata for the media')
     )
+    can_download = models.BooleanField(
+        _('can download'),
+        db_index=True,
+        default=False,
+        help_text=_('Media has a matching format and can be downloaded')
+    )
     downloaded = models.BooleanField(
         _('downloaded'),
         db_index=True,
@@ -538,14 +544,6 @@ class Media(models.Model):
                     return False
         return False
 
-    @property
-    def can_download(self):
-        '''
-            Returns boolean True if the media can be downloaded, that is, the media
-            has stored formats which are compatible with the source requirements.
-        '''
-        return self.get_format_str() is not False
-
     @property
     def loaded_metadata(self):
         try:

app/sync/signals.py

@@ -46,6 +46,10 @@ def source_post_save(sender, instance, created, **kwargs):
             queue=str(instance.pk),
             verbose_name=verbose_name.format(instance.name)
         )
+    # Trigger the post_save signal for each media item linked to this source as various
+    # flags may need to be recalculated
+    for media in Media.objects.filter(source=instance):
+        media.save()
 
 
 @receiver(pre_delete, sender=Source)
@@ -91,6 +95,16 @@ def media_post_save(sender, instance, created, **kwargs):
                 queue=str(instance.source.pk),
                 verbose_name=verbose_name.format(instance.name)
             )
+    # Recalculate the "can_download" flag, this may need to change if the source
+    # specifications have been changed
+    if instance.get_format_str():
+        if not instance.can_download:
+            instance.can_download = True
+            instance.save()
+    else:
+        if instance.can_download:
+            instance.can_download = True
+            instance.save()
 
 
 @receiver(pre_delete, sender=Media)