Mongodb aggrgation function for sum of embedded documents not working - mongodb

Question

i get an error like
Array
(
[errmsg] => exception: the $unwind field path must be specified as a string
[code] => 15981
[ok] => 0
)
While i used following query for given embedded document (I want to sum of rate_number from my given record of table structure)
global $DB, $mongo;
$theObjId = new MongoId($post_id);
$collection = $mongo->getCollection('mongo_hw_posts');
$rt_sum = $collection->aggregate(
array('$unwind'=>$rate),
array('$group'=>
array(
'_id' => $theObjId
),
array(
'rate_number'=>array('$sum' =>$rate.'rate_number')
))
);
table structure
{
"_id": ObjectId("51ff3b38636e3b9803000001"),
"class_id": NumberInt(2986),
"created_by": NumberInt(1758),
"created_datetime": NumberInt(1375681336),
"deleted": NumberInt(0),
"learn": {
"0": {
"user_id": NumberInt(0),
"learn_date": NumberInt(0)
}
},
"parent_id": "0",
"post_text": "2%20C",
"post_type": "text_comment",
"rate": {
"0": {
"user_id": NumberInt(0),
"rate_date": NumberInt(0),
"rate_number": NumberInt(0)
},
"1": {
"user_id": NumberInt(1457),
"rate_date": NumberInt(1375764137),
"rate_number": NumberInt(3)
},
"2": {
"user_id": NumberInt(1619),
"rate_date": NumberInt(1375764694),
"rate_number": NumberInt(8)
}
},
"serialized_data": "",
"unique_key": "8bdddfe8137d14702b4517f7e8e88ee3",
"user_role": "student"
}

Answer 1

There are a few things wrong with your aggrgation command.
$unwind
Instead of:
array( '$unwind' => $rate ),
You need to use:
array( '$unwind' => '$rate'),
$rate is not just a PHP variable, but a field-value expression in MongoDB.
But you can't use the $unwind like this either, because of:
"errmsg" : "exception: Value at end of $unwind field path '$rate' must be an Array, but is a Object",
That's because rate is:
"rate": {
"0": {
"user_id": NumberInt(0),
"rate_date": NumberInt(0),
"rate_number": NumberInt(0)
},
"1": {
"user_id": NumberInt(1457),
"rate_date": NumberInt(1375764137),
"rate_number": NumberInt(3)
},
"2": {
"user_id": NumberInt(1619),
"rate_date": NumberInt(1375764694),
"rate_number": NumberInt(8)
}
}
But it needs to look like:
"rate": [
{
"user_id": NumberInt(0),
"rate_date": NumberInt(0),
"rate_number": NumberInt(0)
},
{
"user_id": NumberInt(1457),
"rate_date": NumberInt(1375764137),
"rate_number": NumberInt(3)
},
{
"user_id": NumberInt(1619),
"rate_date": NumberInt(1375764694),
"rate_number": NumberInt(8)
}
]
Otherwise, $unwind will not work. You need to change your documents for this.
$group
Your $group is also wrong, instead of:
array('$group'=>
array(
'_id' => $theObjId
),
array(
'rate_number'=>array('$sum' =>$rate.'rate_number')
)
)
You need to make it syntax wise:
array('$group'=>
array(
'_id' => $theObjId
'rate_number'=>array('$sum' => '$rate.rate_number')
)
)
I don't understand why you have:
'_id' => $theObjId
Are you trying to only summarize the rates for one post? If that's the case, you will need to add a $match and change the $theObjId to null, something like this:
$rt_sum = $collection->aggregate(
array( '$match' => array( '_id' => $theObjId ) ),
array( '$unwind' => '$rate' ),
array( '$group'=>
array(
'_id' => null,
'rate_number' => array('$sum' => '$rate.rate_number')
)
)
);
The full example is here:
<?php
$m = new MongoClient;
$c = $m->test->so;
$c->drop();
$post_id = "51ff3b38636e3b9803000001";
$theObjId = new MongoId($post_id);
$c->insert( array(
"_id" => new MongoId("51ff3b38636e3b9803000001"),
"class_id" => 2986,
"created_by" => 1758,
"created_datetime" => 1375681336,
"deleted" => 0,
"learn" => array(
array(
"user_id" => 0,
"learn_date" => 0
)
),
"parent_id" => "0",
"post_text" => "2%20C",
"post_type" => "text_comment",
"rate" => array(
array(
"user_id" => 0,
"rate_date" => 0,
"rate_number" => 0
),
array(
"user_id" => 1457,
"rate_date" => 1375764137,
"rate_number" => 3
),
array(
"user_id" => 1619,
"rate_date" => 1375764694,
"rate_number" => 8
)
),
"serialized_data" => "",
"unique_key" => "8bdddfe8137d14702b4517f7e8e88ee3",
"user_role" => "student"
) );
$rt_sum = $c->aggregate(
array( '$match' => array( '_id' => $theObjId ) ),
array( '$unwind' => '$rate' ),
array( '$group'=>
array(
'_id' => null,
'rate_number' => array('$sum' => '$rate.rate_number')
)
)
);
var_dump ($rt_sum);
And the output is:
array(2) {
'result' =>
array(1) {
[0] =>
array(2) {
'_id' => NULL
'rate_number' => int(11)
}
}
'ok' =>
double(1)
}