Added index on AbstractImage.created_at

Fixes #1598

Speeds up image index/chooser when many images exist in the site

wagtail/tests/testapp/migrations/0007_auto_20150819_0614.py

@@ -0,0 +1,24 @@
+# -*- coding: utf-8 -*-
+from __future__ import unicode_literals
+
+from django.db import models, migrations
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('tests', '0006_image_file_size'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='customimagewithadminformfields',
+            name='created_at',
+            field=models.DateTimeField(db_index=True, verbose_name='Created at', auto_now_add=True),
+        ),
+        migrations.AlterField(
+            model_name='customimagewithoutadminformfields',
+            name='created_at',
+            field=models.DateTimeField(db_index=True, verbose_name='Created at', auto_now_add=True),
+        ),
+    ]

wagtail/wagtailimages/migrations/0008_image_created_at_index.py

@@ -0,0 +1,19 @@
+# -*- coding: utf-8 -*-
+from __future__ import unicode_literals
+
+from django.db import models, migrations
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('wagtailimages', '0007_image_file_size'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='image',
+            name='created_at',
+            field=models.DateTimeField(db_index=True, verbose_name='Created at', auto_now_add=True),
+        ),
+    ]

wagtail/wagtailimages/models.py

@@ -63,7 +63,7 @@ class AbstractImage(models.Model, TagSearchable):
     file = models.ImageField(verbose_name=_('File'), upload_to=get_upload_to, width_field='width', height_field='height')
     width = models.IntegerField(verbose_name=_('Width'), editable=False)
     height = models.IntegerField(verbose_name=_('Height'), editable=False)
-    created_at = models.DateTimeField(verbose_name=_('Created at'), auto_now_add=True)
+    created_at = models.DateTimeField(verbose_name=_('Created at'), auto_now_add=True, db_index=True)
     uploaded_by_user = models.ForeignKey(settings.AUTH_USER_MODEL, verbose_name=_('Uploaded by user'), null=True, blank=True, editable=False)
 
     tags = TaggableManager(help_text=None, blank=True, verbose_name=_('Tags'))