Entity insert/delete/update hooks are missing

Sorry this is so last minute, only realised this after a quick chat in irc last night with chx and Damien. I'll try to summarize the issues:

http://drupal.org/project/mongodb is going to provide non-SQL field storage.

field_attach_query() doesn't let you do things like get a list of nodes ordered by creation date with a certain field value - you can't join between SQL and non-SQL. This is why we have the taxonomy_index and forum_index tables in core (since SQL-SQL joins aren't exactly fast either).

So, mongodb field storage is very likely to store the entire entity in mongodb, so that lists etc. can be done with mongodb nifty query syntax.

There's also a mongodb caching backend.

Any site wanting to store fields out of SQL is likely to also want to use http://drupal.org/project/entitycache - since that also reduces hits to SQL.

entitycache also stores the entire entity object in whatever caching backend is supplied.

To avoid double storage of the same information in mongodb (triple storage if you count the entity data in SQL), it would make sense for mongodb to depend on entity cache so that we only store the information once - then it could be got via cache_get(), field_attach_query(), custom queries, or a views backend from the same place.

However entitycache relies on special casing core entities and implementing their specific hooks to handle cache setting, clearing etc. That's not acceptable for a field storage backend which needs to be able to handle any field attached to any arbitrary entity.

field_attach_presave() runs way before other hooks have run, so you don't have the complete entity - it's therefore only suitable for dealing with field values, not the full entity as it should be loaded.

Therefore, non-SQL field storage backends and/or entity cache, need to be able to implement an entity level hook which has access to the full object on insert/update and need to be able to respond to delete.

Probably the easiest way to do this is to add entity_invoke($op, $entity_type, $entity); calls in the right places.

I'm about to upload a patch for update/insert. Delete is tricky for two reasons:

1. hook_user_cancel() is by no means an equivalent for other _delete() hooks

2. It looks like field_attach_delete() is extremely inconsistently applied - it's likely that would suffice (although being inconsistent) since deletion is a more straightforward operation than insert/update - you have an ID, you delete stuff. This probably needs its own issue :/

I'm posting this as a critical bug report, because if the above summary is correct, pluggable field storage isn't currently possible in HEAD with non-SQL backends which provide some kind of query syntax (i.e. anything no-SQL-like).