NAME
MongoDB::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 age33>>-
$db->users->find({age => {'$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 age33 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 age30>>-
$db->users->find({"age" => {'$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
If you do something in the MongoDB shell and you would like to translate it to Perl, the best way is to run the function in the shell without parentheses, which will print the source. You can then generally translate the source into Perl fairly easily.
For example, suppose we want to use db.foo.validate in Perl. We could run:
> db.foo.validate
function (full) {
var cmd = {validate:this.getName()};
if (typeof full == "object") {
Object.extend(cmd, full);
} else {
cmd.full = full;
}
var res = this._db.runCommand(cmd);
if (typeof res.valid == "undefined") {
res.valid = false;
var raw = res.result || res.raw;
if (raw) {
var str = "-" + tojson(raw);
res.valid = !(str.match(/exception/) || str.match(/corrupt/));
var p = /lastExtentSize:(\d+)/;
var r = p.exec(str);
if (r) {
res.lastExtentSize = Number(r[1]);
}
}
}
return res;
}Thus, we can translate the important parts into Perl:
$db->run_command({validate => "foo"});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).
QUERYING
Nested Fields
MongoDB allows you to store deeply nested structures and then query for fields within them using dot-notation. For example, suppose we have a users collection with documents that look like:
{
"userId" => 12345,
"address" => {
"street" => "123 Main St",
"city" => "Springfield",
"state" => "MN",
"zip" => "43213"
}
}If we want to query for all users from Springfield, we can do:
my $cursor = $users->find({"address.city" => "Springfield"});This will search documents for an "address" field that is a subdocument and a "city" field within the subdocument.
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".