Mongo Map Reduce first time

Ok, I've come up with something that I think may do what you want. Note, that this may not work exactly since I'm not 100% sure of your schema (considering your examples show refer available in type a, but not b (I'm not sure if that's an omission, or what considering you want to view by referer)... Anyway, here's what I've come up with:

The map function:

function() {    var obj = {        "types": {},        "tags": {},    }    obj.types[this.type] = 1;    if (this.tags) {        for (var tag in this.tags) {            obj.tags[this.tags[tag]] = 1;        }    }    emit(this.refer.url, obj);}

The Reduce function:

function(key, values) {    var obj = {        "types": {},        "tags": {},    }    for (var i = 0; i < values.length; i++) {        for (var type in values[i].types) {            if (!type in obj.types) {                obj.types[type] = 0;            }            obj.types[type] += values[i].types[type];        }        for (var tag in values[i].tags) {            if (!tag in obj.tags) {                obj.tags[tag] = 0;            }            obj.tags[tag] += values[i].tags[tag];        }    }    return obj;}

So basically, how it works is this. The Map function uses a key of refer.url (what I guessed based on your description). So the end result will look like an array with _id equal to refer.url (It groups based on url). It then creates an object that has two objects under it (types and tags). The reason for the object is so that map and reduce can emit the same format object. Other than that, I THINK that it should be relatively self explanatory (If you don't understand, I can try to explain more)...

So let's implement this in PHP (Assuming that $map and $reduce are strings with the above contained with them for terseness):

$mapFunc = new MongoCode($map);$reduceFunc = new MongoCode($reduce);$query = array(    'time' => array('$gte' => time() - (60*60*60*24*30)),    'refer.external' => true);$collection = 'visits';$command = array(    'mapreduce' => $collection,    'map' => $mapFunc,    'reduce' => $reduceFunc,    'query' => $query,);$statsInfo = $db->command($command);$statsCollection = $db->selectCollection($sales['result']);$stats = $statsCollection->find();foreach ($stats as $stat) {    echo $stats['_id'] .' Visited ';    foreach ($stats['value']['types'] as $type => $times) {        echo "Type $type $times Times, ";    }    foreach ($stats['value']['tags'] as $tag => $times) {        echo "Tag $tag $times Times, ";    }    echo "\n";}

Note, I haven't tested this. This is just what I've come up with based on my understanding of your schema, and from my understanding of Mongo and its Map-Reduce implementation...

Map reduce is already implemented in Mongo DB ODM:

http://www.doctrine-project.org/docs/mongodb_odm/1.0/en/reference/map-reduce.html