Storing and retrieving data with MongoDB

By OK - February 26, 2017

Storing and retrieving data with MongoDB

> show dbs
local 0.03125GB
test (empty)
> show collections
startup_log
system.indexes
users

> db.createCollection('users')

>db.users.insert({'name':'root','uid':'0001'})

>db.users.find()

{ "_id" : ObjectId("58b334b1f4d36fb19595a7a9"), "name" : "root", "uid" : "0001" }

In this recipe, we will look at basic CRUD operations with MongoDB. We will learn how to create databases, store, retrieve, and update stored data. This is a recipe to get started with MongoDB.

Getting ready

Make sure that you have installed and configured MongoDB. You can also use the MongoDB installation on a remote server.

How to do it…

Follow these steps to store and retrieve data with MongoDB:

Open a shell to interact with the Mongo server:
```
$ mongo
```
To open a shell on a remote server, use the command given. Replace server_ip and port with the respective values:
```
$ mongo server_ip:port/db
```
To create and start using a new database, type use dbname. Since schemas in MongoDB are dynamic, you do not need to create a database before using it:
```
> use testdb
```
You can type help in Mongo shell to get a list of available commands and help regarding a specific command:
> help: Let’s insert our first document:
```
> db.users.insert({‘name’:’ubuntu’,’uid’:1001})
```
To view the created database and collection, use the following commands:
```
> show dbs
```
```
> show collections
```

You can also insert multiple values for a key, for example, which groups a user belongs to:

> db.users.insert({‘name’:’root’,’uid’:1010, ‘gid’:[1010, 1000, 1111]})

Check whether a document is successfully inserted:
```
> db.users.find()
```
To get a single record, use findOne():
```
> db.users.findOne({uid:1010})
```
To update an existing record, use the update command as follows:
```
> db.users.update({name:’ubuntu’}, {$set:{uid:2222}})
```
To remove a record, use the remove command. This will remove all records with a name equal to ubuntu:
```
> db.users.remove({‘name’:’ubuntu’})
```
To drop an entire collection, use the drop() command:
```
> db.users.drop()
```
To drop a database, use the dropDatabase() command:
```
> db.users.dropDatabase()
```

How it works…

The preceding examples show very basic CRUD operations with the MongoDB shell interface. MongoDB shell is also a JavaScript shell. You can execute all JS commands in a MongoDB shell. You can also modify the shell with the configuration file, ~/.mongorc.js. Similar to shell, MongoDB provides language-specific drivers, for example, MongoDB PHP drivers to access MongoDB from PHP.

MongoDB works on the concept of collections and documents. A collection is similar to a table in MySQL and a document is a set of key value stores where a key is similar to a column in a MySQL table. MongoDB does not require any schema definitions and accepts any pair of keys and values in a document. Schemas are dynamically created. In addition, you do not need to explicitly create the collection. Simply type a collection name in a command and it will be created if it does not already exist. In the preceding example, users is a collection we used to store all data. To explicitly create a collection, use the following command:

> use testdb
> db.createCollection(‘users’)

You may be missing the where clause in MySQL queries. We have already used that with the findOne() command:

> db.users.findOne({uid:1010})

You can use $lt for less than, $lte for less than or equal to, $gt for greater than, $gte for greater than or equal to, and $ne for not equal:

> db.users.findOne({uid:{$gt:1000}})

In the preceding example, we have used the where clause with the equality condition uid=1010. You can add one more condition as follows:

> db.users.findOne({uid:1010, name:’root’})

To use the or condition, you need to modify the command as follows:

> db.users.find ({$or:[{name:’ubuntu’}, {name:’root’}]})

You can also extract a single key (column) from the entire document. The find command accepts a second optional parameter where you can specify a select criteria. You can use values 1 or 0. Use 1 to extract a specific key and 0 otherwise:

> db.users.findOne({uid:1010}, {name:1})

> db.users.findOne({uid:1010}, {name:0})

There’s more…

You can install a web interface to manage the MongoDB installation. There are various open source web interfaces listed on Mongo documentation at http://docs.mongodb.org/ecosystem/tools/administration-interfaces/.

When you start a mongo shell for the first time, you may see a warning message regarding transperent_hugepage and defrag. To remove those warnings, add the following lines to /etc/init/mongod.conf, below the $DAEMONUSER /var/run/mongodb.pid line:

if test -f /sys/kernel/mm/transparent_hugepage/enabled; then
  echo never > /sys/kernel/mm/transparent_hugepage/enabled
fi
if test -f /sys/kernel/mm/transparent_hugepage/defrag; then
  echo never > /sys/kernel/mm/transparent_hugepage/defrag
fi

Find more details on this Stack Overflow post at http://stackoverflow.com/questions/28911634/how-to-avoid-transparent-hugepage-defrag-warning-from-mongodb

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