MongoDB

Prerequisites

• Start the mongoDB server using the command mongod in the terminal
• Start the mongoDB interactive shell using the command mongo on a separate terminal.

NOTE : Sometimes the mongod server is already running in the background in ubuntu. So first just try running the command mentioned in the second step i.e. mongo. If it doesn't work then start the server and the interactive shell, one after another respectively.

After successfully entering the interactive shell try out the following stuff :

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Contents

• CREATE
  • Creating a database
  • List databases
  • Creating collections
  • Inserting documents
• READ
  • Filters
  • Comparison Operators
  • Logical Operators
  • Functions
  • Miscellaneous
• UPDATE
• DELETE
  • Deleting documents
  • Deleting collections
  • Deleting databases
• Resources

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CREATE

Creating a database

• To create a database enter the command use db_name where db_name is the name of the database you want to create.
• This command is also used to switch between databases
• Eg:-

  use cars

List databases

• To view the various databases created enter :-

  show dbs

• Result:

  admin   0.000GB
  config  0.000GB
  local   0.000GB
  temp    0.000GB
  

NOTE : The cars db created in the earlier command isn't visible as it has no data in it. Once a collection is created it will be visible on executing the above command.

• To view the database you are currently using enter:-

  db

Creating collections

• Collections are in mongoDB what tables are in SQL.
• The only difference is that you don't need to have a defined structure of the column names and the datatype those columns will have in mongoDB unlike SQL.
• You can directly create a collection without specifying any details of the data that will be entered in it.
• This can be done using the command db.createCollection('collection_name')
• Eg:-

  db.createCollection('car_details')

Inserting documents

• In mongoDB when you make an entry in a collection that entry is called a document i.e. a row in terms of SQL
• You can insert a document in a collection using the command db.collection_name.insert(data)
• Structure of the data in the example below :-
  • brand
  • model_name
  • type
  • engine
    • fuel
    • cc
    • bhp
  • color
  • safety
    • airbag (if airbags isn't present then this field is omitted else its type(i.e. airbag type, for eg:- front/front and rear) is associated with the key)
    • sb_warn (seatbelt warning)
• Eg:-

  db.car_details.insert(
      {
          'brand': 'chevrolet',
          'model_name': 'beat',
          'type': 'hatchback',
          'engine': {
              'fuel': 'petrol',
              'cc': 1000,
              'bhp': 100
          },
          'color': ['red', 'blue', 'green'],
          'safety': {
              'sb_warn': false
          },
          'price' : 400000
      }
  )

• You can insert multiple entries in one go by using an array of objects i.e.

  db.collection_name.insert([{},{},....])

• Copy and paste the contents of this file in your terminal.
  • The file contains an array of objects containing the car_details.

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READ

• Fetching data from the documents is done using the find() function.

• Eg:-

  db.car_details.find()
  

• This returns all the documents in the car_details collection as no parameter was passed as to which filter(s) should be applied.

• To get a readable format of the results use the pretty() function

• Eg: -

  db.car_details.find().pretty()

• This returns a properly formatted result of the find() output.

Filters

• To get the document(s) based on some filters enter the key and the corresponding value as the first parameter to the find() function.
• Eg:-
• Here we want to get the details of the cars of type sedan

  db.car_details.find({ 'type' : 'sedan' }).pretty()

• You could relate this type of filtering to the WHERE clause in SQL.

Comparison Operators

• Comparison operators can be used to filter documents in monogDB using the following specifiers:-
  • $gt = greater than
  • $gte = greater than equal to
  • $lt = less than
  • $lte = less than equal to
  • $ne = not equal to
• Eg:- ($lte)

  db.car_details.find({
      'price' : { '$lte' : 500000 }
  }).pretty()

Logical Operators

• Logical operators can be used to filter documents in mongoDB using the following specifications:-

  • $or = OR operation
  • $and = AND operation

• Eg:- (AND)

  db.car_details.find({'$and' : [
      { 'type' : 'sedan' },
      { 'price' : { '$gte' : 500000 } }
  ]}).pretty()

  Notice that the following command will also output the same result as the previous, this is because the default operation in case of multiple filters is an AND operation.

• Eg:- (AND)(without the $and specifier)

  db.car_details.find({
       'type' : 'sedan' ,
       'price' : { '$gte' : 500000 }
  }).pretty()

• Eg:- (OR)

  db.car_details.find({'$or': [
      { 'type' : 'sedan' },
      { 'price' : { '$gte' : 500000 } }
  ]}).pretty()

Functions

• sort()

  • Sort function can be used to sort the result of the query based on a key
  • For ascending set the value of the key to 1 and for descending set it to -1
  • Eg:- SORT (desc)

    db.car_details.find().sort(
        {'price' : -1}
    ).pretty()

• limit()

  • To limit the no. of results of the query, send the no of results to be displayed as a parameter to the limit function
  • Eg:-

    db.car_details.find().limit(3).pretty()

• count()

  • To just display the no of results that the query will output the count() function can be used
  • Eg:-

    db.car_details.find().count()

Miscellaneous

• $exists

  • To check whether a key exists in the documents in the collections the $exists specifier
  • Eg:-

    db.car_details.find({
        'safety.airbags': {'$exists' : true}
    }).pretty()

• $regex

  • Regular expressions can also be used to find entries having a pattern using the $regex specifier
  • Eg:-

    db.car_details.find({
        'model_name' : { $regex: /^e/ }
    }).pretty()

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UPDATE

• There are 3 functions associated to updation of documents in mongoDB :-

  • updateOne()

    • This method is used to update the first matching document based on the filters provided
    • Eg:-

      db.car_details.updateOne(
          { 'model_name' : 'cruze' },
          { '$set' : { 'price' : 1500000 } }
      )

  • updateMany()

    • This method is used to update all the matching documents based on the filters provided
    • Eg:-

      db.car_details.updateMany(
          { },
          { '$rename' : { 'body_type' : 'type' } }
      )

  • replaceOne()

    • This method is used to update the first matching document based on the filters provided
    • Eg:-

      db.car_details.replaceOne(
        { 'model_name' : 'jazz' },
        {
            'brand': 'honda',
            'model_name': 'jazz',
            'type': 'hatchback',
            'engine': {
                'fuel': 'diesel',
                'cc': 1150,
                'bhp': 115
            },
            'color': ['silver','orange','white'],
            'safety': {
                'airbags' : 'front',
                'sb_warn': true
            },
            'price' : 750000
        }
      )

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DELETE

Deleting documents

• One or many documents can be deleted in mongoDb using the following commands:-

  • deleteOne()

    • This method deletes the first document that matches the conditions specified
    • Eg:-

      db.car_details.deleteOne(
             { 'model_name' : 'city' }
      )

  • deleteMany()

    • This method deletes the all the documents that match the conditions specified. If no condition is specified then all documents in the collection are deleted.

    • Eg:-

      db.car_details.deleteMany(
             { 'body_type' : 'suv' }
      )

Deleting collections

• To delete a collection enter db.collection_name.drop()
• Eg:-

  db.car_details.drop()

Deleting databases

• To delete the database you are currently working on, enter :-

  db.dropDatabase()

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Resources

• MongoDB official Documentation
• MongoDB Tutorials Point
• Marvels of MongoDB (pluralsight) slides

NOTE : The official documentation is more than sufficient to get a hold of both basic and advanced concepts in mongoDB as it has an exhaustive no. of examples associated with every functionality. So, it is recommended to first have a look at the documentation before searching for a tutorial.