CouchDB - Basic design for joins data - couchdb

Question

Just playing around with CouchDb and CouchApp, what amazing tech ! Very surprised, seams to be very powerful. After playing and reading a lot, as I'm a old relational database user, I still questioning myself about how to design some basic things.
Here is my question :
1/ I have a document of type 'user' and document of type 'item'
2/ My Couchdb contains the following documents :
{ "_id": "...", "type": "user", "email":"u1#gmail.com" ... }
{ "_id": "...", "type": "user", "email":"u2#gmail.com" ... }
{ "_id": "...", "type": "user", "email":"u3#gmail.com" ... }
{ "_id": "...", "type": "user", "email":"u4#gmail.com" ... }
{ "_id": "...", "type": "item", "title":"My title",
created_by:"u1#gmail.com", modified_by:"u3#gmail.com" }
3/ Now I want a view or something to fetch document by type=item and _id with informations for each users (creator & modifier)
I have seen a way to emulate a simple join here : http://www.cmlenz.net/archives/2007/10/couchdb-joins
But I can't adapt it for two joins, I'am playing around with key format since few hours, testing lots things, but nothing works.
I think, I'm missing something important with CouchDb map/reduce, if someone has some help I will appreciate it.
PS : Don't answer me to insert 'user' document inside 'item' document. This not my question.
Relax, relax ... :-)

Answer 1

I think you should put _ids in created_by and modified_by:
{ "_id": "u1", "type": "user", "email":"u1#gmail.com" ... }
{ "_id": "u2", "type": "user", "email":"u2#gmail.com" ... }
{ "_id": "u3", "type": "user", "email":"u3#gmail.com" ... }
{ "_id": "u4", "type": "user", "email":"u4#gmail.com" ... }
{ "_id": "anitem", "type": "item", "title":"My title",
created_by:"u1", modified_by:"u3" }
so you can use the following map function and query it with ?key="anitem"&include_docs=true:
function(doc) {
if (doc.type === "item") {
emit(doc._id, 1);
emit(doc._id, { _id: doc.created_by });
emit(doc._id, { _id: doc.modified_by });
}
}
You can read Jan Lehnardt's post about it for more details.
As a side note, I generally put the type in the _id so it is easier to get unique keys, and you do not need a view if you only want to filter by type:
{ "_id": "user/username1", "email":"u1#gmail.com" ... }
{ "_id": "user/username2", "email":"u2#gmail.com" ... }
{ "_id": "user/username3", "email":"u3#gmail.com" ... }
{ "_id": "user/username4", "email":"u4#gmail.com" ... }
{ "_id": "item/itemid1", "title":"My title",
created_by:"user/username1", modified_by:"user/username3" }
and the map function is
function(doc) {
if (doc._id.slice(0, 4) === "item/") {
emit(doc._id, 1);
emit(doc._id, { _id: doc.created_by });
emit(doc._id, { _id: doc.modified_by });
}
}
UPDATE: Due to bug COUCHDB-1229, use of / in doc._id may cause problems. Depending on your use-case, it may be better to use another separator, e.g. : or _.

Answer 2

What do you mean here "two joins"? Is this "tableA join tableB ON ... AND ..."?
Now I want a view or something to fetch document by type=item and _id with informations for each users
This can be done without two/more joins.
Anyway, my advise is to divide your data in 2 databases: items & users. The example above suits only few simple tasks. But when your data grows big (say 10K users & 100K items) it becomes very hard to process your data and it's kinda sucks that all your documents differ from each other with only one field.