How to count document elements inside a mongo collection with php? - mongodb

Question

I have the following structure of a mongo document:
{
"_id": ObjectId("4fba2558a0787e53320027eb"),
"replies": {
"0": {
"email": ObjectId("4fb89a181b3129fe2d000000"),
"sentDate": "2012-05-21T11: 22: 01.418Z"
}
"1": {
"email": ObjectId("4fb89a181b3129fe2d000000"),
"sentDate": "2012-05-21T11: 22: 01.418Z"
}
"2" ....
}
}
How do I count all the replies from all the documents in the collection?
Thank you!

Answer 1

In the following answer, I'm working with a simple data set with five replies across the collection:
> db.foo.find()
{ "_id" : ObjectId("4fba6b0c7c32e336fc6fd7d2"), "replies" : [ 1, 2, 3 ] }
{ "_id" : ObjectId("4fba6b157c32e336fc6fd7d3"), "replies" : [ 1, 2 ] }
Since we're not simply counting documents, db.collection.count() won't help us here. We'll need to resort to MapReduce to scan each document and aggregate the reply array lengths. Consider the following:
db.foo.mapReduce(
function() { emit('totalReplies', { count: this.replies.length }); },
function(key, values) {
var result = { count: 0 };
values.forEach(function(value) {
result.count += value.count;
});
return result;
},
{ out: { inline: 1 }}
);
The map function (first argument) runs across the entire collection and emits the number of replies in each document under a constant key. Mongo will then consider all emitted values and run the reduce function (second argument) a number of times to consolidate (literally reduce) the result. Hopefully the code here is straightforward. If you're new to map/reduce, one caveat is that the reduce method must be capable of processing its own output. This is explained in detail in the MapReduce docs linked above.
Note: if your collection is quite large, you may have to use another output mode (e.g. collection output); however, inline works well for small data sets.
Lastly, if you're using MongoDB 2.1+, we can take advantage of the Aggregation Framework to avoid writing JS functions and make this even easier:
db.foo.aggregate(
{ $project: { replies: 1 }},
{ $unwind: "$replies" },
{ $group: {
_id: "result",
totalReplies: { $sum: 1 }
}}
);
Three things are happening here. First, we tell Mongo that we're interested in the replies field. Secondly, we want to unwind the array so that we can iterate over all elements across the fields in our projection. Lastly, we'll tally up results under a "result" bucket (any constant will do), adding 1 to the totalReplies result for each iteration. Executing this query will yield the following result:
{
"result" : [{
"_id" : "result",
"totalReplies" : 5
}],
"ok" : 1
}
Although I wrote the above answers with respect to the Mongo client, you should have no trouble translating them to PHP. You'll need to use MongoDB::command() to run either MapReduce or aggregation queries, as the PHP driver currently has no helper methods for either. There's currently a MapReduce example in the PHP docs, and you can reference this Google group post for executing an aggregation query through the same method.

Answer 2

I haven't checked your code, might work as well. I've did the following and it just works:
$replies = $db->command(
array(
"distinct" => "foo",
"key" => "replies"
)
);
$all = count($replies['values']);

Answer 3

I've did it again using the group command of the PHP Mongo Driver. It's similar to a MapReduce command.
$keys = array("replies.type" => 1); //keys for group by
$initial = array("count" => 0); //initial value of the counter
$reduce = "function (obj, prev) { prev.count += obj.replies.length; }";
$condition = array('replies' => array('$exists' => true), 'replies.type' => 'follow');
$g = $db->foo->group($keys, $initial, $reduce, $condition);
echo $g['count'];
Thanks jmikola for giving links to Mongo.

Answer 4

JSON should be
{
"_id": ObjectId("4fba2558a0787e53320027eb"),
"replies":[
{
0: {
"email": ObjectId("4fb89a181b3129fe2d000000"),
"sentDate": "2012-05-21T11: 22: 01.418Z"
},
1: {
"email": ObjectId("4fb89a181b3129fe2d000000"),
"sentDate": "2012-05-21T11: 22: 01.418Z"
},
2: {....}
]
}