Storing lists of data into memcached can mean either storing a single item with a serialized array, or trying to manipulate a huge “collection” of data by adding, removing items without operating on the whole set. Both should be possible.
One thing to keep in mind is memcached’s 1 megabyte limit on item size, so storing the whole collection (ids, data) into memcached might not be the best idea.
Steven Grimm explains a better approach on the mailing list: http://lists.danga.com/pipermail/memcached/2007-July/004578.html
Following the link gives this quote:
A better way to deal with this kind of thing is with a two-phase fetch. So instead of directly caching an array of event data, instead cache an array of event IDs. Query that list, then use it construct a list of the keys of individual event objects you want to fetch, then multi-get that list of keys.
…Another advantage of a scheme like this is that you can update an item’s data without having to read then write every list that contains that item. Just update it by ID (like you’d do in your database queries) and all the lists that contain it will magically get the correct information.
That always feels nice.