Add a filter_spec field on Rendition to track the filter spec string used

This is an initial step towards retiring Filter as a model (#2881). We
cannot go further than this in the present release, as we want to retain
the ability for developers with custom image models to use the Django
migration autodetector - this implies that all schema changes pertaining
to a Wagtail point release must happen in one go (there's no possibility
of applying a schema migration, then a data migration, then another schema
migration).

wagtail/wagtailimages/migrations/0014_add_filter_spec_field.py

@@ -0,0 +1,26 @@
+# -*- coding: utf-8 -*-
+# Generated by Django 1.10 on 2016-08-11 12:03
+from __future__ import unicode_literals
+
+from django.db import migrations, models
+import django.db.models.deletion
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('wagtailimages', '0013_make_rendition_upload_callable'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='rendition',
+            name='filter_spec',
+            field=models.CharField(blank=True, db_index=True, max_length=255, null=True),
+        ),
+        migrations.AlterField(
+            model_name='rendition',
+            name='filter',
+            field=models.ForeignKey(blank=True, null=True, on_delete=django.db.models.deletion.CASCADE, related_name='+', to='wagtailimages.Filter'),
+        ),
+    ]

wagtail/wagtailimages/migrations/0015_fill_filter_spec_field.py

@@ -0,0 +1,43 @@
+# -*- coding: utf-8 -*-
+# Generated by Django 1.10 on 2016-08-11 13:25
+from __future__ import unicode_literals
+
+from django.db import migrations
+
+
+def fill_filter_spec_forward(apps, schema_editor):
+    # Populate Rendition.filter_spec with the spec string of the corresponding Filter object
+    Rendition = apps.get_model('wagtailimages', 'Rendition')
+    db_alias = schema_editor.connection.alias
+    for rendition in Rendition.objects.using(db_alias).select_related('filter'):
+        rendition.filter_spec = rendition.filter.spec
+        rendition.save()
+
+
+def fill_filter_spec_reverse(apps, schema_editor):
+    # Populate the Rendition.filter
+    Rendition = apps.get_model('wagtailimages', 'Rendition')
+    Filter = apps.get_model('wagtailimages', 'Filter')
+    db_alias = schema_editor.connection.alias
+
+    filters_by_spec = {}
+    for rendition in Rendition.objects.using(db_alias):
+        try:
+            filter = filters_by_spec[rendition.filter_spec]
+        except KeyError:
+            filter, _ = Filter.objects.get_or_create(spec=rendition.filter_spec)
+            filters_by_spec[rendition.filter_spec] = filter
+
+        rendition.filter = filter
+        rendition.save()
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('wagtailimages', '0014_add_filter_spec_field'),
+    ]
+
+    operations = [
+        migrations.RunPython(fill_filter_spec_forward, fill_filter_spec_reverse),
+    ]

wagtail/wagtailimages/models.py

@@ -458,7 +458,8 @@ class Filter(models.Model):
 
 
 class AbstractRendition(models.Model):
-    filter = models.ForeignKey(Filter, related_name='+')
+    filter = models.ForeignKey(Filter, related_name='+', null=True, blank=True)
+    filter_spec = models.CharField(max_length=255, db_index=True, null=True, blank=True)
     file = models.ImageField(upload_to=get_rendition_upload_to, width_field='width', height_field='height')
     width = models.IntegerField(editable=False)
     height = models.IntegerField(editable=False)
@@ -505,6 +506,12 @@ class AbstractRendition(models.Model):
         filename = self.file.field.storage.get_valid_name(filename)
         return os.path.join(folder_name, filename)
 
+    def save(self, *args, **kwargs):
+        # populate the `filter_spec` field with the spec string of the filter. In Wagtail 1.8
+        # Filter will be dropped as a model, and lookups will be done based on this string instead
+        self.filter_spec = self.filter.spec
+        return super(AbstractRendition, self).save(*args, **kwargs)
+
     class Meta:
         abstract = True
 

wagtail/wagtailimages/tests/test_models.py

@@ -207,6 +207,12 @@ class TestRenditions(TestCase):
         self.assertEqual(rendition.width, 400)
         self.assertEqual(rendition.height, 300)
 
+        # check that the rendition has been recorded under the correct filter,
+        # via both the Rendition.filter_spec attribute (which will come into action
+        # in Wagtail 1.8) and the linked Filter model (which will be retired in 1.8)
+        self.assertEqual(rendition.filter_spec, 'width-400')
+        self.assertEqual(rendition.filter.spec, 'width-400')
+
     def test_resize_to_max(self):
         rendition = self.image.get_rendition('max-100x100')