Question

I am trying to run the following code in PHP to query a MongoDB:

<?

$m = new Mongo(); // connect

$dogs = $m->dogs;
$races = $dogs->newdogs;

$js = "function() {
    return this.location == 'SHEFFIELD'
}";

$dataSet = $races->find(array('$where' => $js));

foreach ($dataSet as $r){

}

?>

When I run this and watch the console, I see the query being run once.

When I change the foreach loop to be nested within another one like this:

foreach(range(1,5) as $test){
    foreach ($dataSet as $r){

    }
}

I see the query being run 7 times in the console?
Is this something stupid I am doing? A scoping issue? Or am I just misunderstanding how MongoDB is supposed to work?

Thanks
AH

Answer 1

This happens because $dataSet is a MongoCursor, not an array. A MongoCursor is a representation of a query. It will be turned into an array “on-demand”, that means that when you use foreach on it, $dataSet is converted into an array by simply querying.

Since you do it within another loop, the MongoCursor is executed every time it encounters the foreach. If you don’t want that behaviour, you can use iterator_to_array, since a MongoCursor is just an iterator:

$executed = iterator_to_array($dataSet); // Actual query execution
foreach($executed as $r) {               // Iterate the array, not the iterator
    // Hic sunt ponies
}

EDIT: Keep in mind that iterator_to_array converts the entire result set into an array in memory. If you have a very big result set, this can cause huge and unnecessary memory consumption. It’s advisable to stick with a single foreach call, since it will only load one single row into memory at once.