MongoDB::Examples(3) User Contributed Perl Documentation MongoDB::Examples(3)NAMEMongoDB::Examples - Some examples of MongoDB syntax
MAPPING SQL TO MONGODB
For developers familiar with SQL, the following chart should help you
see how many common SQL queries could be expressed in MongoDB.
These are Perl-specific examples of translating SQL queries to
MongoDB's query language. To see the JavaScript (or other languages')
mappings, see <http://dochub.mongodb.org/core/sqlToMongo>.
"CREATE TABLE USERS (a Number, b Number)" Implicit, can be done
explicitly.
"INSERT INTO USERS VALUES(1,1)"
$db->users->insert({a => 1, b => 1});
"SELECT a,b FROM users"
$db->users->find({}, {a => 1, b => 1});
"SELECT * FROM users"
$db->users->find;
"SELECT * FROM users WHERE age=33"
$db->users->find({age => 33})
"SELECT a,b FROM users WHERE age=33"
$db->users->find({age => 33}, {a => 1, b => 1});
"SELECT * FROM users WHERE age=33 ORDER BY name"
$db->users->find({age => 33})->sort({name => 1});
"<SELECT * FROM users WHERE age"33>>
$db->users->find({age => {'$gt' => 33}})
"<SELECT * FROM users WHERE age<33">
$db->users->find({age => {'$lt' => 33}})
"SELECT * FROM users WHERE name LIKE "%Joe%""
$db->users->find({name => qr/Joe/});
"SELECT * FROM users WHERE name LIKE "Joe%""
$db->users->find({name => qr/^Joe/});
"<SELECT * FROM users WHERE age"33 AND age<=40>>
$db->users->find({age => {'$gt' => 33, '$lte' => 40}});
"SELECT * FROM users ORDER BY name DESC"
$db->users->find->sort({name => -1});
"CREATE INDEX myindexname ON users(name)"
$db->users->ensure_index({name => 1});
"CREATE INDEX myindexname ON users(name,ts DESC)"
$db->users->ensure_index(Tie::IxHash->new(name => 1, ts => -1));
"SELECT * FROM users WHERE a=1 and b='q'"
$db->users->find({a => 1, b => "q"});
"SELECT * FROM users LIMIT 10 SKIP 20"
$db->users->find->limit(10)->skip(20);
"SELECT * FROM users WHERE a=1 or b=2"
$db->users->find({'$or' => [{a => 1}, {b => 2}]});
"SELECT * FROM users LIMIT 1"
$db->users->find->limit(1);
"EXPLAIN SELECT * FROM users WHERE z=3"
$db->users->find({z => 3})->explain;
"SELECT DISTINCT last_name FROM users"
$db->run_command({distinct => "users", key => "last_name"});
"SELECT COUNT(*y) FROM users"
$db->users->count;
"<SELECT COUNT(*y) FROM users where age " 30>>
$db->users->find({"age" => {'$gt' => 30}})->count;
"SELECT COUNT(age) from users"
$db->users->find({age => {'$exists' => 1}})->count;
"UPDATE users SET a=1 WHERE b='q'"
$db->users->update({b => "q"}, {'$set' => {a => 1}});
"UPDATE users SET a=a+2 WHERE b='q'"
$db->users->update({b => "q"}, {'$inc' => {a => 2}});
"DELETE FROM users WHERE z="abc""
$db->users->remove({z => "abc"});
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"
]
};
Find-and-modify
The find-and-modify command is similar to update (or remove), but it
will return the modified document. It can be useful for implementing
queues or locks.
For example, suppose we had a list of things to do, and we wanted to
remove the highest-priority item for processing. We could do a "find"
in MongoDB::Collection and then a "remove" in MongoDB::Collection, but
that wouldn't be atomic (a write could occur between the query and the
remove). Instead, we can use find and modify.
my $next_task = $db->run_command({
findAndModify => "todo",
sort => {priority => -1},
remove => 1
});
This will atomically find and pop the next-highest-priority task.
See <http://www.mongodb.org/display/DOCS/findAndModify+Command> for
more details on find-and-modify.
Group
The group command is similar to "GROUP BY" in SQL. You can use the
"run_command" in MongoDB::Database method to perform group-bys with
MongoDB.
For example, suppose we have a number of local businesses stored in a
"business" collection. If we wanted to find the number of coffeeshops
in each neighborhood, we could do:
my $reduce = <<REDUCE;
function(doc, prev) {
for (var t in doc.tags) {
if (doc.tags[t] == "coffeeshop") {
prev["num coffeeshops"]++;
break;
}
}
}
REDUCE
my $result = $db->run_command({group => {
'ns' => "business",
'key' => {"neighborhood" => 1},
'initial' => {"num coffeeshops" => 0},
'$reduce' => MongoDB::Code->new(code => $reduce)
This would return something like:
{
'ok' => '1',
'keys' => 4,
'count' => '487', # total number of documents
'retval' => [
{
'neighborhood' => 'Soho',
'num coffeeshops' => '23'
},
{
'neighborhood' => 'Chinatown',
'num coffeeshops' => '14'
},
{
'neighborhood' => 'Upper East Side',
'num coffeeshops' => '10'
},
{
'neighborhood' => 'East Village',
'num coffeeshops' => '87'
}
]
};
Thus, there are 23 coffeeshops in Soho, 14 in Chinatown, and so on.
See <http://www.mongodb.org/display/DOCS/Aggregation> for more details
on grouping.
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".
perl v5.14.2 2011-08-29 MongoDB::Examples(3)
