MongoDB Shell (mongosh) basics - Step-by-Step Execution

Concept Flow - MongoDB Shell (mongosh) basics

Start mongosh

↓

Connect to MongoDB

↓

Show databases

↓

Select database

↓

Show collections

↓

Run queries or commands

↓

Exit mongosh

This flow shows how you start the MongoDB shell, connect to the server, explore databases and collections, run commands, and then exit.

Execution Sample

MongoDB

mongosh
show dbs
db = db.getSiblingDB('testdb')
db.createCollection('users')
db.users.insertOne({name: 'Alice'})
db.users.find()

This code starts the shell, lists databases, switches to 'testdb', creates a collection, inserts a document, and finds documents.

Execution Table

Step	Command	Action	Result
1	mongosh	Start MongoDB shell	Connected to MongoDB server
2	show dbs	List all databases	[admin, local, testdb]
3	db = db.getSiblingDB('testdb')	Switch to 'testdb' database	Switched to 'testdb'
4	db.createCollection('users')	Create 'users' collection	Collection 'users' created
5	db.users.insertOne({name: 'Alice'})	Insert document into 'users'	Inserted 1 document with _id
6	db.users.find()	Find all documents in 'users'	[{_id: ObjectId(...), name: 'Alice'}]
7	exit	Exit mongosh	Shell closed

💡 User exits the shell with 'exit' command

Variable Tracker

Variable	Start	After Step 3	After Step 4	After Step 5	After Step 6	Final
db	default	testdb	testdb	testdb	testdb	testdb
users collection	none	none	created	created with 1 document	created with 1 document	created with 1 document

Key Moments - 3 Insights

Why do we use db.getSiblingDB('testdb') instead of just db?

Because 'db' starts as the default database (usually 'test'), getSiblingDB switches to the named database without restarting the shell. See execution_table step 3.

What does insertOne return and why is _id important?

insertOne returns an object with the inserted document's _id, which uniquely identifies the document. This is shown in execution_table step 5.

Why does find() return an array with ObjectId?

Because find() returns documents matching the query; each document has a unique _id of type ObjectId. See execution_table step 6.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of 'db' after step 3?

A 'testdb'

B 'admin'

C 'local'

D 'default'

💡 Hint

Check the variable_tracker table under 'db' after Step 3

At which step is the 'users' collection created?

A Step 2

B Step 4

C Step 3

D Step 5

💡 Hint

Look at the 'Action' column in execution_table for collection creation

If you run db.users.find() before inserting any documents, what would the result be?

A A document with _id

B An error

C An empty array []

D The collection is deleted

💡 Hint

Think about what find() returns when no documents exist in the collection

Concept Snapshot

MongoDB Shell (mongosh) basics:
- Start shell with 'mongosh'
- Use 'show dbs' to list databases
- Switch DB with db.getSiblingDB('name')
- Create collections with createCollection()
- Insert documents with insertOne()
- Query documents with find()
- Exit shell with 'exit'

Full Transcript

This lesson shows how to use the MongoDB shell called mongosh. First, you start the shell by typing 'mongosh'. Then, you can list all databases using 'show dbs'. To work with a specific database, use db.getSiblingDB('databaseName'). You can create a collection with createCollection('collectionName'). To add data, use insertOne() with a document. To see data, use find(). Finally, exit the shell by typing 'exit'. Each step changes the state of the shell and database, which we tracked in the tables.

Practice

1. What command in mongosh lists all the databases available on the server?

easy

A. show dbs

B. list databases

C. show collections

D. dbs list

Solution

1. Step 1: Understand the command purpose

   The question asks for the command that lists all databases in mongosh.

2. Step 2: Recall mongosh commands

   show dbs is the correct command to list databases. Other options are incorrect or do not exist.

3. Final Answer:

   show dbs -> Option A

4. Quick Check:

   List databases = show dbs [OK]

Hint: Use 'show dbs' to list all databases quickly [OK]

Common Mistakes:

• Confusing 'show collections' with listing databases
• Using non-existent commands like 'list databases'
• Trying 'dbs list' which is invalid

2. Which of the following is the correct syntax to switch to a database named shop in mongosh?

easy

A. switch shop

B. use shop

C. db switch shop

D. change db shop

Solution

1. Step 1: Identify the command to change database

   In mongosh, the command to switch databases is use followed by the database name.

2. Step 2: Match the correct syntax

   use shop correctly switches to the 'shop' database. Other options are invalid commands.

3. Final Answer:

   use shop -> Option B

4. Quick Check:

   Switch database = use [OK]

Hint: Remember: 'use' changes the current database [OK]

Common Mistakes:

• Using 'switch' instead of 'use'
• Adding extra words like 'db' or 'change'
• Typing commands that don't exist in mongosh

3. Given the following commands in mongosh:

use testdb
db.products.insertOne({name: 'Pen', price: 1.5})
db.products.find({name: 'Pen'}).toArray()

What will be the output of the find() command?

medium

A. [{ _id: ObjectId("...") , name: 'Pen', price: 1.5 }]

B. null

C. SyntaxError

D. []

Solution

1. Step 1: Insert a document into 'products' collection

   The insertOne() command adds a document with name 'Pen' and price 1.5 to the 'products' collection.

2. Step 2: Query the collection for documents with name 'Pen'

   The find() command searches for documents matching {name: 'Pen'}. Since we inserted one, it returns an array with that document including an auto-generated _id.

3. Final Answer:

   [{ _id: ObjectId("...") , name: 'Pen', price: 1.5 }] -> Option A

4. Quick Check:

   Inserted doc found = array with document [OK]

Hint: Inserted documents appear in find results as arrays [OK]

Common Mistakes:

• Expecting find() to return null if document exists
• Confusing syntax causing errors
• Thinking find() returns a single object, not an array

4. You run this command in mongosh:

db.users.insertOne({name: 'Alice', age: 30})

But get an error: ReferenceError: db is not defined. What is the most likely cause?

medium

A. The syntax of the insertOne command is incorrect

B. The insertOne method does not exist

C. You forgot to switch to a database using use command

D. MongoDB server is not running

Solution

1. Step 1: Understand the error message

   ReferenceError: db is not defined means the shell does not know what db is.

2. Step 2: Identify cause in mongosh context

   This usually happens if you have not selected a database with use. Without a current database, db is undefined.

3. Final Answer:

   You forgot to switch to a database using use command -> Option C

4. Quick Check:

   db undefined = forgot 'use' [OK]

Hint: Always run 'use dbname' before using db commands [OK]

Common Mistakes:

• Assuming insertOne method is missing
• Thinking syntax is wrong when it is correct
• Ignoring the need to start MongoDB server

5. You want to list all collections in the current database and then insert a document into a collection named orders. Which sequence of commands in mongosh is correct?

hard

A. show collections
use orders
db.insertOne({item: 'Book', qty: 3})

B. show dbs
db.orders.insertOne({item: 'Book', qty: 3})

C. use orders
show collections
db.insertOne({item: 'Book', qty: 3})

D. show collections
db.orders.insertOne({item: 'Book', qty: 3})

Solution

1. Step 1: List collections in current database

   show collections lists all collections in the current database. This is the first correct step.

2. Step 2: Insert document into 'orders' collection

   Use db.orders.insertOne(...) to insert a document into the 'orders' collection. This is the correct syntax.

3. Final Answer:

   show collections db.orders.insertOne({item: 'Book', qty: 3}) -> Option D

4. Quick Check:

   List collections then insert = show collections db.orders.insertOne({item: 'Book', qty: 3}) [OK]

Hint: Use 'db.collection.insertOne()' after 'show collections' [OK]

Common Mistakes:

• Trying to 'use' a collection as a database
• Calling insertOne directly on db without collection
• Listing databases instead of collections