Inverted Index possible from couchdb view?

Question

Suppose I have couchdb docs that look like so:

{
    "_id": "id",
    "_rev": "rev",
    "title": "foobar",
    "URI": "http://www.foobar.com",
    "notes": "",
    "date": 1334177254774,
    "tags": [
        "tag1",
        "tag2",
        "tag3"
    ],
    "date_modified": 1334177278457,
    "deleted": false
}

What I want is to create an inverted index from the tags, so I end up with something like:

{
    "tag1": [
        _id,
        _id,
        _id
    ],
    "tag2": [
        _id,
        _id,
        ...
    ]
}

From what I've read and tried, couchdb might not let me do it. I can't get it done in the map phase, and it doesn't seem like I can do it in the couch reduce phase. Is this something I need to accomplish in another layer of the app?

Answer 1

You can achieve this with the map feature of CouchDB.

CouchDB likes tall lists, not fat lists. So, to "cut with the grain" on this problem, you want a view keyed on the tag, with one row per document ID.

// View rows (conceptual diagram)
// Key  , Value
[ "tag1", "_id1"
, "tag1", "_id2"
, "tag1", "_id3"

, "tag2", "_id2"
, "tag2", "_id4"
, "tag3", "_id6"
]

To get a list of all document IDs with a tag, hit the view with a tag name as the key, GET /db/_design/ddoc/_view/tags?key="tag1".

This should be enough. But as lagniappe, you can set the "reduce" value to "_count" to get a count of all tags used (useful for building a tag cloud). Just remember to add &reduce=false when you want to query your index.