That would be a very cool feature to have in couch! How hard do you think it would be to write a patch that accomplished this the correct way?
On Mon, Apr 19, 2010 at 12:45 PM, J Chris Anderson <[email protected]>wrote: > > On Apr 18, 2010, at 12:01 PM, Michael Genereux wrote: > > > Reposting this from the user list. Figured it belongs in the dev list. > > > > Sorry to leave you hanging for so long. (Vacation weekend!) > > There are 2 reasons CouchDB doesn't have this feature. The important one is > that it could easily give new users the wrong idea. Coming from a RDBMS > background you might expect something like this to be transactional. In > Couch, it wouldn't be. > > Generally in Couch we've pushed work to the database client. When the > client is running on the same box as CouchDB, there would be 0 savings in > processing overhead to run these transforms on the server, vs using a > application script. > > The second reason is the amount of code it would require. If someone wants > to write it, there's a decent chance it'd be accepted as a patch (if it were > done right.) > > Here's how I think it could be implemented (actually a more generic feature > that can be used for lots of things.) > > A design document could have a ddoc.changes function, which subscribes to > the changes feed of the database it runs in. The changes function could then > do anything it wanted, like send an email anytime a doc is saved that has a > doc.send_me_as_email = true field. > > Your use case can be accomplished by having a function that watches to see > if any of the docs have a price that hasn't yet been updated, and change the > price. So it could load the document, change the price, and set a flag on > the document that says doc.price_changed = true. It would ignore any > documents that already had price_changed. > > This is almost exactly how you would accomplish this at the application > level. The only reason to pull such an operation into the design document is > that CouchDB would take care of keeping it running, and that there would be > a standard way to author a changes listener. > > I'm not sure the status of it (node.js is a moving target) but Mikeal has > something much like this already on github: > > http://github.com/mikeal/node.couch.js/ > > I'd suggest understanding how this works (and why it's like it is) before > thinking about extending CouchDB. The non-transactional nature of CouchDB > means you need to understand the _changes feed before you can think about > doing anything "complete" like updating all docs that match a given pattern. > > Chris > > > > I was doing some ad-hoc UPDATEs and DELETEs on a SQL database the > > other day and it crossed my mind, how could I do the same on CouchDB? > > I don't want to write an application to do something so simple. It > > seems to me that I should be able to produce a Javascript views, both > > named and temporary, that in turn produce results that get imported > > into the CouchDB Bulk Document API as a single transaction. > > > > Here's the update example for a 10% increase in prices: > > function( doc ) { > > doc.price = doc.price * 1.1; > > emit( null, doc ) > > } > > > > Here's the delete example for old records: > > function( doc ) { > > if( doc.year == 2009 ) { > > doc._deleted = true; > > emit( null, doc ) > > } > > } > > > > Also, much like INSERT ... SELECT notation, this could be used to copy > > records. No need for the non-http compliant COPY method that does the > > same on a single record level. Changes can be made to the copy on the > > fly. Very efficient since the duplication and the update occur in one > > transaction. > > > > Example to duplicate all 'foo' widgets to a set of 'bar' widgets: > > function( doc ) { > > if( doc.widget_type == 'foo' ) { > > reset( doc ); // helper function that does: > > // delete( doc._id ); delete( doc._rev ); > > // and any other special vars in future > > doc.widget_type = 'bar' > > emit( null, doc ) > > } > > } > > > > I like not having to learn another command, so I reused emit even > > though bulk api won't use the first field. If this feature existed, > > this is how I would have expected to use it knowing that the key > > parameter has no value in this use of a view. > > > > I would love some feedback on this. > > > > Michael > >
