Collections vs tables mental model in MongoDB - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to find all documents in the collection.

MongoDB

db.users.[1]()

Drag options to blanks, or click blank then click option'

Afind

Bdelete

Cupdate

Dinsert

Attempts:

3 left

💡 Hint

Common Mistakes

Using insert() instead of find() to get data.

Trying to update or delete when just reading.

The find() method retrieves all documents from a MongoDB collection, similar to selecting all rows from a table in SQL.

2fill in blank

medium

Complete the code to insert a new document into the collection.

MongoDB

db.products.[1]({ name: 'Pen', price: 1.5 })

Drag options to blanks, or click blank then click option'

Afind

BdeleteOne

CupdateOne

DinsertOne

Attempts:

3 left

💡 Hint

Common Mistakes

Using find() instead of insertOne() to add data.

Using deleteOne() or updateOne() by mistake.

The insertOne() method adds a single new document to a collection, similar to inserting a new row in a table.

3fill in blank

hard

Fix the error in the code to update a document's price.

MongoDB

db.items.updateOne({ name: 'Book' }, { [1]: { price: 12 } })

Drag options to blanks, or click blank then click option'

A$add

B$get

C$set

D$remove

Attempts:

3 left

💡 Hint

Common Mistakes

Using $get or $add instead of $set.

Forgetting the dollar sign in the operator.

The $set operator updates the value of a field in a document. Other operators like $get or $add are not valid for updates.

4fill in blank

hard

Fill both blanks to delete all documents where age is less than 18.

MongoDB

db.students.deleteMany({ age: { [1]: [2] } })

Drag options to blanks, or click blank then click option'

A$lt

B$gt

C18

D21

Attempts:

3 left

💡 Hint

Common Mistakes

Using $gt (greater than) instead of $lt.

Using wrong age number.

The $lt operator means 'less than'. So this query deletes documents where age is less than 18.

5fill in blank

hard

Fill all three blanks to find documents where status is 'active' and sort by score descending.

MongoDB

db.players.find({ status: [1] }).sort({ [2]: [3] })

Drag options to blanks, or click blank then click option'

A'active'

Bscore

C-1

D1

Attempts:

3 left

💡 Hint

Common Mistakes

Forgetting quotes around 'active'.

Using 1 instead of -1 for descending sort.

The query finds documents with status 'active' and sorts them by score in descending order using -1.

Practice

1. Which of the following best describes a MongoDB collection compared to a SQL table?

easy

A. A collection requires a strict schema like SQL tables.

B. A collection stores data in fixed rows and columns.

C. A collection stores flexible documents without fixed columns.

D. A collection cannot store multiple records.

Solution

1. Step 1: Understand SQL tables

   SQL tables store data in rows and columns with a fixed schema, meaning each row has the same columns.

2. Step 2: Understand MongoDB collections

   MongoDB collections store documents that can have different fields and structures, so they are flexible and schema-less.

3. Final Answer:

   A collection stores flexible documents without fixed columns. -> Option C

4. Quick Check:

   Collections = flexible documents [OK]

Hint: Collections are flexible; tables have fixed columns [OK]

Common Mistakes:

• Thinking collections have fixed columns like tables
• Assuming collections require strict schemas
• Believing collections cannot hold multiple documents

2. Which of the following is the correct way to create a collection named users in MongoDB?

easy

A. db.createCollection('users');

B. db.users.create();

C. CREATE TABLE users;

D. CREATE COLLECTION users;

Solution

1. Step 1: Recall MongoDB syntax for creating collections

   MongoDB uses the method db.createCollection('name') to create a collection explicitly.

2. Step 2: Check other options

   CREATE COLLECTION users; and C use SQL syntax, which is invalid in MongoDB. db.users.create(); is not a valid MongoDB command.

3. Final Answer:

   db.createCollection('users'); -> Option A

4. Quick Check:

   MongoDB collection creation = db.createCollection() [OK]

Hint: Use db.createCollection('name') to create collections [OK]

Common Mistakes:

• Using SQL syntax like CREATE TABLE
• Trying to call create() on collection name
• Omitting quotes around collection name

3. Given a MongoDB collection products with documents:

{"name": "Pen", "price": 1.5}

{"name": "Notebook", "price": 3.0, "color": "blue"}

What will db.products.find({}) return?

medium

A. All documents including fields like name, price, and color if present.

B. Only documents with the price field.

C. Only documents with the color field.

D. An error because documents have different fields.

Solution

1. Step 1: Understand find({}) query

   The query find({}) returns all documents in the collection regardless of their fields.

2. Step 2: Check document fields

   Documents can have different fields in MongoDB collections; this does not cause errors or filtering.

3. Final Answer:

   All documents including fields like name, price, and color if present. -> Option A

4. Quick Check:

   find({}) returns all documents [OK]

Hint: find({}) returns all documents regardless of fields [OK]

Common Mistakes:

• Thinking find({}) filters by fields
• Expecting errors due to different document structures
• Assuming only documents with certain fields are returned

4. You tried to insert documents with different fields into a MongoDB collection but got an error. What is the most likely cause?

medium

A. MongoDB collections require all documents to have the same fields.

B. MongoDB does not allow inserting multiple documents.

C. You forgot to create the collection before inserting.

D. You used SQL INSERT syntax instead of MongoDB insert methods.

Solution

1. Step 1: Recall MongoDB insert syntax

   MongoDB uses insertOne() or insertMany() methods, not SQL INSERT statements.

2. Step 2: Understand document flexibility

   MongoDB collections allow documents with different fields; this does not cause errors.

3. Final Answer:

   You used SQL INSERT syntax instead of MongoDB insert methods. -> Option D

4. Quick Check:

   MongoDB insert = insertOne()/insertMany() [OK]

Hint: Use MongoDB insertOne()/insertMany(), not SQL INSERT [OK]

Common Mistakes:

• Assuming MongoDB requires fixed schema
• Not creating collection first (MongoDB auto-creates collections)
• Believing MongoDB forbids multiple inserts

5. You want to migrate a SQL table with fixed columns to MongoDB. Which approach best fits MongoDB's collection model?

hard

A. Keep the exact SQL schema and enforce it strictly in MongoDB.

B. Store each row as a document with the same fields, but allow optional fields for future flexibility.

C. Convert the table into multiple collections, each with one column as documents.

D. Store all rows as a single large document with arrays for each column.

Solution

1. Step 1: Understand MongoDB document flexibility

   MongoDB collections store documents that can have different fields, so keeping the same fields but allowing optional ones fits well.

2. Step 2: Evaluate other options

   Convert the table into multiple collections, each with one column as documents. breaks data into many collections unnecessarily. Keep the exact SQL schema and enforce it strictly in MongoDB. tries to enforce strict schema, which MongoDB does not require. Store all rows as a single large document with arrays for each column. stores all data in one document, which is inefficient and not recommended.

3. Final Answer:

   Store each row as a document with the same fields, but allow optional fields for future flexibility. -> Option B

4. Quick Check:

   MongoDB collections = flexible documents per row [OK]

Hint: Migrate rows as flexible documents with optional fields [OK]

Common Mistakes:

• Trying to enforce strict SQL schema in MongoDB
• Splitting columns into separate collections
• Storing all rows in one big document