Introduce touched_at DateTime property

[Sibyx]

Currently to fill <updated> of the OPDS catalog we use the updated_at property of objects (Catalog, Feed, Entry). This is not a good idea, because the updated_at property reflects the last change of the particular object in terms of ORM. This value is not updated if a new entry was added to the catalog. This kind of behavior may confuse the consumers of the API because feeds look inactive.

Solution

My solution to the problem is to implement touched_at property for these models:

• Catalog,
• Feed,
• Entry.

This property should be updated accordingly to the catalog hierarchy if something "dramatically" changes, for example:

• new acquisition was created,
• new entry was created.

Implementation considerations

Django Signals looks pretty good for handling required events.

Property can be stored inside the database or we can use Redis as a cache server for such value (access is supposed to be faster in case of rendering catalog).