PHP and MongoDB - The Definitive Guide to MongoDB - page 111

Similarly, you can get only the last three e-mail addresses in the list by making the integer negative, as shown in
the following example:
// Connect to the database
$c = new MongoClient();
// Select the collection 'people' from the database 'contacts'
$collection = $c->contacts->people;
// Specify our search operator
$query = array("Last Name" => "Moran");
// Specify the conditional operator
$cond = (object)array('E-Mail' => array('$slice' => -3));
// Execute the query and store it under the $cursor variable
$cursor = $collection->find($query, $cond);
// For each document it finds within the collection, print the contents
while ($document = $cursor->getNext())
{
print_r($document);
}
Or, you can skip the first two entries and limit the results to three:
// Connect to the database
$c = new MongoClient();
// Select the collection 'people' from the database 'contacts'
$collection = $c->contacts->people;
// Specify our search operator
$query = array("Last Name" => "Moran");
// Specify the conditional operator
$cond = (object)array('E-Mail' => array('$slice' => array(2, 3)));
// Execute the query and store it under the $cursor variable
$cursor = $collection->find($query, $cond);
// For each document it finds within the collection, print the contents
while ($document = $cursor->getNext())
{
print_r($document);
}
The $slice operator is a great method for limiting the number of items in an array; you'll definitely want to keep
this operator in mind when programming with the MongoDB driver and PHP.
Determining Whether a Field Has a Value
You can use the $exists operator to return a result based on whether a field holds a value (regardless of the value of
this field). As illogical as this may sound, it's actually very handy. For example, you can search for contacts where the
Age field has not been set yet; or you can search for contacts for whom you have a street name.
The following example returns any contacts that do not have an Age field set:
// Connect to the database
$c = new MongoClient();
// Select the collection 'people' from the database 'contacts'
$collection = $c->contacts->people;