NAME

MongoDB::Examples - Some more advanced examples

DATABASE COMMANDS

Distinct

The distinct command returns all values for a given key in a collection. For example, suppose we had a collection with the following documents (_id value ignored):

{ 'name' => 'a', code => 1 }
{ 'name' => 'b', code => 1 }
{ 'name' => 'c', code => 2 }
{ 'name' => 'd', code => "3" }

If we wanted to see all of values in the "code" field, we could run:

my $result = $db->run_command([
   "distinct" => "collection_name",
   "key"      => "code",
   "query"    => {}
]);

Notice that the arguments are in an array, to ensure that their order is preserved. You could also use a Tie::IxHash.

query is an optional argument, which can be used to only run distinct on specific documents. It takes a hash (or Tie::IxHash or array) in the same form as "find($query)" in MongoDB::Collection.

Running distinct on the above collection would give you:

{
    'ok' => '1',
    'values' => [
                  1,
                  2,
                  "3"
                ]
};

MapReduce

MapReduce is a powerful aggregation tool. (For traditional queries, you should use MongoDB::Collection::query.)

This example counts the number of occurences of each tag in a collection. Each document contains a "tags" array that contains zero or more strings.

my $map = <<MAP;
function() {
    this.tags.forEach(function(tag) {
        emit(tag, {count : 1});
    });
}
MAP

my $reduce = <<REDUCE;
function(prev, current) {
    result = {count : 0};
    current.forEach(function(item) {
        result.count += item.count;
    });
    return result;
}
REDUCE

my $cmd = Tie::IxHash->new("mapreduce" => "foo",
    "map" => $map,
    "reduce" => $reduce);

my $result = $db->run_command($cmd);

See the MongoDB documentation on MapReduce for more information (http://dochub.mongodb.org/core/mapreduce).

UPDATING

Positional Operator

In MongoDB 1.3.4 and later, you can use positional operator, $, to update elements of an array. For instance, suppose you have an array of user information and you want to update a user's name.

A sample document in JavaScript:

{
    "users" : [
        {
            "name" : "bill",
            "age" : 60
        },
        {
            "name" : "fred",
            "age" : 29
        },
    ]
}

The update:

$coll->update({"users.name" => "fred"}, {'users.$.name' => "george"});

This will update the array so that the element containing "name" => "fred" now has "name" => "george".