Deprecate the use of Filter as an ORM model

Warnings are thrown on calling save() and accessing Filter.objects

wagtail/wagtailimages/models.py

@@ -269,7 +269,7 @@ class AbstractImage(CollectionMember, index.Indexed, models.Model):
 
     def get_rendition(self, filter):
         if isinstance(filter, string_types):
-            filter, created = Filter.objects.get_or_create(spec=filter)
+            filter = Filter(spec=filter)
 
         cache_key = filter.get_cache_key(self)
         Rendition = self.get_rendition_model()
@@ -365,6 +365,7 @@ def image_delete(sender, instance, **kwargs):
     instance.file.delete(False)
 
 
+# RemovedInWagtail19Warning: We will remove the models.Model
 class Filter(models.Model):
     """
     Represents one or more operations that can be applied to an Image to produce a rendition
@@ -482,6 +483,28 @@ class Filter(models.Model):
 
         cls._registered_operations = dict(operations)
 
+    def save(self, *args, **kwargs):
+        warnings.warn(
+            "Filter.save() is deprecated; Filter will no longer be an ORM model in Wagtail 1.9. "
+            "Instantiate and use it in-memory instead",
+            RemovedInWagtail19Warning, stacklevel=2
+        )
+        return super(Filter, self).save(*args, **kwargs)
+
+
+class WarnOnManagerAccess(object):
+    def __get__(self, obj, objtype=None):
+        warnings.warn(
+            "Filter.objects is deprecated; Filter will no longer be an ORM model in Wagtail 1.9. "
+            "Instantiate and use it in-memory instead",
+            RemovedInWagtail19Warning, stacklevel=2
+        )
+        return objtype._objects
+
+
+Filter._objects = Filter.objects
+Filter.objects = WarnOnManagerAccess()
+
 
 class AbstractRendition(models.Model):
     filter = models.ForeignKey(Filter, related_name='+', null=True, blank=True)  # DEPRECATED - RemovedInWagtail19Warning

wagtail/wagtailimages/templatetags/wagtailimages_tags.py

@@ -74,8 +74,7 @@ class ImageNode(template.Node):
 
     @cached_property
     def filter(self):
-        _filter, _ = Filter.objects.get_or_create(spec=self.filter_spec)
-        return _filter
+        return Filter(spec=self.filter_spec)
 
     def render(self, context):
         try: