MongoDB Notes & Reference

by Corey Snyder

I'm going through the tutorials over at Mongodb.org in order to jump-start my knowledge of MongoDB. This is where I will keep my MongoDB notes and references. Feel free to bookmark this page in order to use it as your own personal reference! If you have no idea what MongoDB is, you should start with the Introduction. If you're wondering how to install MongoDB on windows, start here: How to install MongoDB on Windows.

Links:

SQL to Mongo Mapping Chart

Show Collections:

show collections

Show DBS

show dbs

Selecting your Database:

use mydb

Inserting Data into a Collection:

j = { name: "corey" };
db.mydb.save(j);

Retrieving Data:

>db.mydb.find();
{ "_id" : ObjectId("4c2209f9f3924d31102bd84a"), "name" : "corey" }

Adding data in a loop:

> for (var i = 1; i  db.things.find();
{ "_id" : ObjectId("4c2209f9f3924d31102bd84a"), "name" : "mongo" }
{ "_id" : ObjectId("4c2209fef3924d31102bd84b"), "x" : 3 }
{ "_id" : ObjectId("4c220a42f3924d31102bd856"), "x" : 4, "j" : 1 }
...

Cursor Object:

By Default the shell limits the # to 20 when automatically iterating a cursor. Use the "it" shortcut to show the next set of results. 
> db.things.find();
{ "_id" : ObjectId("4c2209f9f3924d31102bd84a"), "name" : "mongo" }
{ "_id" : ObjectId("4c2209fef3924d31102bd84b"), "x" : 3 }
{ "_id" : ObjectId("4c220a42f3924d31102bd856"), "x" : 4, "j" : 1 }
{ "_id" : ObjectId("4c220a42f3924d31102bd857"), "x" : 4, "j" : 2 }
{ "_id" : ObjectId("4c220a42f3924d31102bd858"), "x" : 4, "j" : 3 }
{ "_id" : ObjectId("4c220a42f3924d31102bd859"), "x" : 4, "j" : 4 }
has more

>it

{ "_id" : ObjectId("4c220a42f3924d31102bd868"), "x" : 4, "j" : 19 }

{ "_id" : ObjectId("4c220a42f3924d31102bd869"), "x" : 4, "j" : 20 }

Assigning the cursor object:

> var cursor = db.things.find();
> while (cursor.hasNext()) printjson(cursor.next());
{ "_id" : ObjectId("4c2209f9f3924d31102bd84a"), "name" : "mongo" }
{ "_id" : ObjectId("4c2209fef3924d31102bd84b"), "x" : 3 }
{ "_id" : ObjectId("4c220a42f3924d31102bd856"), "x" : 4, "j" : 1 }
{ "_id" : ObjectId("4c220a42f3924d31102bd857"), "x" : 4, "j" : 2 }
{ "_id" : ObjectId("4c220a42f3924d31102bd858"), "x" : 4, "j" : 3 }
{ "_id" : ObjectId("4c220a42f3924d31102bd859"), "x" : 4, "j" : 4 }
{ "_id" : ObjectId("4c220a42f3924d31102bd85a"), "x" : 4, "j" : 5 }
{ "_id" : ObjectId("4c220a42f3924d31102bd85b"), "x" : 4, "j" : 6 }
{ "_id" : ObjectId("4c220a42f3924d31102bd85c"), "x" : 4, "j" : 7 }
{ "_id" : ObjectId("4c220a42f3924d31102bd85d"), "x" : 4, "j" : 8 }
.....

Using forEach:

> db.things.find().forEach(printjson);
{ "_id" : ObjectId("4c2209f9f3924d31102bd84a"), "name" : "mongo" }
{ "_id" : ObjectId("4c2209fef3924d31102bd84b"), "x" : 3 }
...

Treating Cursors like an array

When using a cursor this way, note that all values up to the highest accessed (cursor[4] above) are loaded into RAM at the same time. This is inappropriate for large result sets, as you will run out of memory. Cursors should be used as an iterator with any query which returns a large number of elements. 
> var cursor = db.things.find();
> printjson(cursor[4]);

{ "_id" : ObjectId("4c220a42f3924d31102bd858"), "x" : 4, "j" : 3 }

SQL Query Examples

SELECT * FROM things WHERE name="mongo"

> db.things.find({name:"mongo"}).forEach(printjson);
{ "_id" : ObjectId("4c2209f9f3924d31102bd84a"), "name" : "mongo" }

SELECT * FROM things WHERE x=4

> db.things.find({x:4}).forEach(printjson);
{ "_id" : ObjectId("4c220a42f3924d31102bd856"), "x" : 4, "j" : 1 }
{ "_id" : ObjectId("4c220a42f3924d31102bd857"), "x" : 4, "j" : 2 }
...

SELECT j FROM things WHERE x=4

Note that the "_id" field is always returned. 
> db.things.find({x:4}, {j:true}).forEach(printjson);
{ "_id" : ObjectId("4c220a42f3924d31102bd856"), "j" : 1 }
{ "_id" : ObjectId("4c220a42f3924d31102bd857"), "j" : 2 }
{ "_id" : ObjectId("4c220a42f3924d31102bd858"), "j" : 3 }
{ "_id" : ObjectId("4c220a42f3924d31102bd859"), "j" : 4 }
...

findOne()

> printjson(db.things.findOne({name:"mongo"}));
{ "_id" : ObjectId("4c2209f9f3924d31102bd84a"), "name" : "mongo" }
This is more efficient because the client requests a single object from the database, so less work is done by the database and the network. This is the equivalent of find({name:"mongo"}).limit(1). This is the equivalent of find({name:"mongo"}).limit(1).

finding a single document by _id:

> var doc = db.things.findOne({_id:ObjectId("4c2209f9f3924d31102bd84a")});
> doc
{ "_id" : ObjectId("4c2209f9f3924d31102bd84a"), "name" : "mongo" }

Limiting the Result Set via limit()

> db.things.find().limit(3);
{ "_id" : ObjectId("4c2209f9f3924d31102bd84a"), "name" : "mongo" }
{ "_id" : ObjectId("4c2209fef3924d31102bd84b"), "x" : 3 }
{ "_id" : ObjectId("4c220a42f3924d31102bd856"), "x" : 4, "j" : 1 }