Re: when to use another document and when not to?

[Sho Fukamachi]

On 21/07/2008, at 1:56 PM, Dean Landolt wrote:

Or, obviously, I would be delighted if someone could show me how I'm
completely wrong and it is actually possible to do this : )

You can. Complex keys. I put together a little test:

http://dev.deanlandolt.com:5984/test/_view/subscriptions/users

Firstly, I appreciate the effort you put into your reply. Great to be able to see your solution in action there. And I hope you don't mind I replicated it so I could examine it locally : )

The map function just uses a two-level key...

function(doc) {
 if (doc.type == 'user') {
   emit([doc.username,0], doc)
 } else if (doc.type == 'subscription') {
   emit([doc.follower, doc.followee], doc)
 }
}

But all that key is doing is sorting the results, right?

Read this for more details:
http://www.cmlenz.net/archives/2007/10/couchdb-joins

Believe me I've read that about 10 times. I still can't see how to solve the problem.

But yes, you can do joins. You can query this view for just one user simply:

http://dev.deanlandolt.com:5984/test/_view/subscriptions/users?startkey= [%22dlandolt%22]&endkey=[%22dlandolt%22,{}]

function(doc) {
     if (doc.type == 'user') {
       emit(doc.username, doc)
     } else if (doc.type == 'subscription') {
       emit(doc.followee, doc)
     }
  }

And you get the exact same results, sans the sorting.







*That* is a join query and that's what I can't see how to do in CouchDB.



Sho

Again I think I am not making myself clear. If you look at that view, you see it returns 3 rows. The first row is the user whose name you have searched upon. In this case your own. The next two rows are the *subscription* documents, which is not what I am talking about. It is easy to get the subscription documents and followee user document for any given followee username. If you abandon sorting all you need is: This is not the kind of join I meant - in fact this is not a join at all, as I understand them. A proper join would get you the follower *user* documents - not just the subscriptions. As it stands if you wanted, say, the full names of the follower users, you are then faced with an n+1 query to look them up one by one. And same in reverse, if you wanted the full user documents of all followed users, starting with the follower's username. Making things worse is that CouchDB doesn't currently have the ability to do multi-key gets (see previous ML discussion). In other words, with a proper join, starting from the username 'katz', you could get back the *user* documents from both following users "sho" and "dlandolt". The user documents, *not* the subscription docs. This is a bit difficult to discuss without ambiguity (or resorting to SQL queries) so let me put it in terms of a use case "challenge" question: with that current DB, can you write a query that, starting with the username "katz", outputs the *names* (not usernames) of all users following him? Many thanks for the discussion and sorry, again, for not explaining myself properly the first n times... my sincere apologies if my stubborn ignorance is annoying everyone here!

Notice the {} in there -- from what I gather objects are at the bottom of the sort, so this query gets all data related to user dlandolt -- the first result (or any result with the second part of a key ending in 0 based on how I wrote my view), and then everything following are the *subscription* docs

that Damien recommended.

Hope that helps.

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