MapReduce using Command Failed to decode document from the server - php

Question

I have a Test database with a collection called collection:
{
"_id": "576008e5b47a6120c800418d",
"UserID": "Paul",
"Page": "A"
}
I want to record webactivity and use mapreduce to get an outcome like
{
"_id": "Paul",
"value": {
"A": 1,
"B": 0,
"C": 0,
"D": 0,
"E": 0
}
}
For a start I tried a simple code with PHP 7 MongoDB Driver 1.1.7 MapReduce using command which failed to decode document from the server:
<?php
$manager = new MongoDB\Driver\Manager("mongodb://localhost:27017");
$command = new MongoDB\Driver\Command(array(
"mapReduce" => "collection",
"map" => "function() { emit(this.UserID, 1); }",
"reduce" => "function(Users, Pages){".
"return Pages;}",
"out" => "ex"
));
try {
$cursor = $manager->executeCommand('Test.collection', $command);
$response = $cursor->toArray()[0];
} catch(MongoDB\Driver\Exception $e) {
echo $e->getMessage(), "\n";
exit;
}
var_dump($response);
?>
Any ideas will be appreciated thanks.

Answer 1

Not too sure if I would recommend MapReduce for this type of operation, would say the aggregation framework will do the aggregation with better performance since the operations are all done in native code without spawning the code to JavaScript for compiling (in the MapReduce case).
With the aggregation operation, all you would need is a $group pipeline that makes use of the $cond operator which allows you to tranform a logical condition into a value. In this case you'd want to specify the pages as keys and their count as the value, with the documents grouped by the UserID.
Consider running the following aggregation operation in mongo shell:
db.collection.aggregate([
{
"$group": {
"_id": "$UserID",
"A": {
"$sum": {
"$cond": [
{ "$eq": [ "$Page", "A" ] },
1,
0
]
}
},
"B": {
"$sum": {
"$cond": [
{ "$eq": [ "$Page", "B" ] },
1,
0
]
}
},
"C": {
"$sum": {
"$cond": [
{ "$eq": [ "$Page", "C" ] },
1,
0
]
}
},
"D": {
"$sum": {
"$cond": [
{ "$eq": [ "$Page", "D" ] },
1,
0
]
}
},
"E": {
"$sum": {
"$cond": [
{ "$eq": [ "$Page", "E" ] },
1,
0
]
}
}
}
}
])
which will produce the output:
{
"_id": "Paul",
"A": 1,
"B": 0,
"C": 0,
"D": 0,
"E": 0
}
for the above sample document.
For brevity, if suppose you have a list of the pages beforehand, you can dynamically produce the pipeline as follows:
var groupOperation = { "$group": { "_id": "$UserID" } },
pages = ["A", "B", "C", "D", "E"];
pages.forEach(function (page){
groupOperation["$group"][page] = {
"$sum": {
"$cond": [
{ "$eq": [ "$Page", page ] },
1,
0
]
}
};
})
db.collection.aggregate([groupOperation]);
Now, translating this to PHP follows:
<?php
$group_pipeline = [
'$group' => [
'_id' => '$UserID',
'A' => [
'$sum' => [
'$cond' => [ [ '$eq' => [ '$Page', 'A' ] ], 1, 0 ]
]
],
'B' => [
'$sum' => [
'$cond' => [ [ '$eq' => [ '$Page', 'B' ] ], 1, 0 ]
]
],
'C' => [
'$sum' => [
'$cond' => [ [ '$eq' => [ '$Page', 'C' ] ], 1, 0 ]
]
],
'D' => [
'$sum' => [
'$cond' => [ [ '$eq' => [ '$Page', 'D' ] ], 1, 0 ]
]
],
'E' => [
'$sum' => [
'$cond' => [ [ '$eq' => [ '$Page', 'E' ] ], 1, 0 ]
]
]
],
];
$aggregation = $collection->aggregate([ group_pipeline ]);
?>
Should you rather stick to MapReduce, then consider changing the map and reduce functions to :
db.collection.mapReduce(
function() {
var obj = {};
["A", "B", "C", "D", "E"].forEach(function (page){ obj[page] = 0; } );
obj[this.Page] = 1;
emit(this.UserID, obj);
},
function(key, values) {
var obj = {};
values.forEach(function(value) {
Object.keys(value).forEach(function(key) {
if (!obj.hasOwnProperty(key)){
obj[key] = 0;
}
obj[key]++;
});
});
return obj;
},
{ "out": { "inline": 1 } }
)
Which gives the output:
{
"results" : [
{
"_id" : "Paul",
"value" : {
"A" : 1,
"B" : 0,
"C" : 0,
"D" : 0,
"E" : 0
}
}
]
}
Translating the above mapReduce operation to PHP is trivial.