Subset pattern for large documents in MongoDB - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to project only the 'name' field from the documents.

MongoDB

db.collection.find({}, { [1]: 1 })

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

Aname

Bage

Caddress

Demail

Attempts:

3 left

💡 Hint

Common Mistakes

Using 0 instead of 1 to include a field

Forgetting to specify the field name

Including multiple fields when only one is needed

To get only the 'name' field, you specify it with 1 in the projection part of the find query.

2fill in blank

medium

Complete the code to exclude the '_id' field from the results.

MongoDB

db.collection.find({}, { [1]: 0 })

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

Aemail

Bname

C_id

Dage

Attempts:

3 left

💡 Hint

Common Mistakes

Trying to exclude fields by setting them to 1

Not excluding '_id' when it's not needed

Using incorrect field names

Setting '_id' to 0 in the projection excludes it from the returned documents.

3fill in blank

hard

Fix the error in the projection to include 'name' and exclude '_id'.

MongoDB

db.collection.find({}, { [1]: 1, _id: 0 })

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

Aemail

Bname

C_id

Dage

Attempts:

3 left

💡 Hint

Common Mistakes

Trying to include and exclude the same field

Using wrong field names

Not setting the correct values for inclusion/exclusion

You cannot include '_id' and exclude it at the same time. To include 'name' and exclude '_id', specify 'name: 1' and '_id: 0'.

4fill in blank

hard

Fill both blanks to project only 'name' and 'email' fields, excluding '_id'.

MongoDB

db.collection.find({}, { [1]: 1, [2]: 1, _id: 0 })

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

Aname

Bage

Cemail

Daddress

Attempts:

3 left

💡 Hint

Common Mistakes

Including '_id' by mistake

Using 0 instead of 1 for included fields

Forgetting to include all desired fields

To include multiple fields, list each with 1 in the projection. Excluding '_id' is done by setting it to 0.

5fill in blank

hard

Fill all three blanks to project 'name' in uppercase, include 'email', and exclude '_id'.

MongoDB

db.collection.aggregate([{ $project: { [1]: { $toUpper: "$name" }, [2]: 1, [3]: 0 } }])

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

AnameUpper

Bemail

C_id

Dname

Attempts:

3 left

💡 Hint

Common Mistakes

Using the original 'name' field instead of a new field

Not excluding '_id'

Incorrect field names in projection

Use $project to create a new field 'nameUpper' with uppercase 'name', include 'email', and exclude '_id' by setting it to 0.

Practice

1. What is the main purpose of using the subset pattern in MongoDB when working with large documents?

easy

A. To retrieve only specific fields from documents to reduce data size

B. To delete large documents from the collection

C. To create indexes on large documents

D. To merge multiple documents into one

Solution

1. Step 1: Understand subset pattern purpose

   The subset pattern is used to get only the needed parts of large documents instead of the whole document.

2. Step 2: Identify the benefit

   This reduces the amount of data sent over the network and improves performance.

3. Final Answer:

   To retrieve only specific fields from documents to reduce data size -> Option A

4. Quick Check:

   Subset pattern = retrieve specific fields [OK]

Hint: Subset pattern means get only needed fields [OK]

Common Mistakes:

• Thinking it deletes documents
• Confusing with indexing
• Assuming it merges documents

2. Which of the following is the correct syntax to retrieve only the name and age fields from documents in a MongoDB collection named users?

easy

A. db.users.find({name: 1, age: 1})

B. db.users.find({}, {name: 1, age: 1})

C. db.users.find({name: 1, age: 1}, {})

D. db.users.find({}, {name: 0, age: 0})

Solution

1. Step 1: Understand find() parameters

   The first parameter is the query filter (empty {} means all documents), the second is the projection specifying fields to include.

2. Step 2: Identify correct projection syntax

   Using {name: 1, age: 1} includes only these fields in the result.

3. Final Answer:

   db.users.find({}, {name: 1, age: 1}) -> Option B

4. Quick Check:

   Projection with 1 includes fields [OK]

Hint: Use find({}, {field: 1}) to include fields [OK]

Common Mistakes:

• Putting projection in the first argument
• Using 0 instead of 1 to include fields
• Omitting the empty filter {}

3. Given the collection products with documents containing _id, name, price, and details fields, what will be the output of this query?

db.products.find({}, {name: 1, price: 1, _id: 0})

medium

A. Documents with only name and price fields, excluding _id

B. Documents with all fields including _id

C. Documents with only _id field

D. Syntax error due to excluding _id

Solution

1. Step 1: Analyze projection fields

   The projection includes name and price with 1, and excludes _id with 0.

2. Step 2: Understand MongoDB default behavior

   By default, _id is included unless explicitly excluded. Here it is excluded.

3. Final Answer:

   Documents with only name and price fields, excluding _id -> Option A

4. Quick Check:

   Projection includes name, price; excludes _id [OK]

Hint: Use _id:0 to exclude _id field [OK]

Common Mistakes:

• Assuming _id is always included
• Thinking projection excludes all other fields automatically
• Confusing 0 and 1 in projection

4. You wrote this query to get only the title and author fields from books collection:

db.books.find({}, {title: 1, author: 0})

Why does this query cause an error or unexpected result?

medium

A. MongoDB requires _id to be included explicitly

B. Missing filter condition in find()

C. Projection keys must be strings, not identifiers

D. Mixing inclusion and exclusion in projection causes error

Solution

1. Step 1: Check projection rules

   MongoDB does not allow mixing inclusion (1) and exclusion (0) in the same projection except for _id.

2. Step 2: Identify the problem in given projection

   Here, title is included (1) and author is excluded (0), which is invalid.

3. Final Answer:

   Mixing inclusion and exclusion in projection causes error -> Option D

4. Quick Check:

   Projection must be all include or all exclude except _id [OK]

Hint: Use only 1s or only 0s in projection except _id [OK]

Common Mistakes:

• Mixing 1 and 0 in projection fields
• Forgetting _id can be excluded separately
• Assuming filter is mandatory for projection

5. You have a large collection orders with documents containing many fields including customer, items, total, and shipping. You want to fetch only customer and the total fields but exclude _id. Which query correctly applies the subset pattern for best performance?

hard

A. db.orders.find({}, {customer: 0, total: 0, _id: 0})

B. db.orders.find({}, {customer: 1, total: 1, _id: 1})

C. db.orders.find({}, {customer: 1, total: 1, _id: 0})

D. db.orders.find({}, {customer: 1, total: 0, _id: 0})

Solution

1. Step 1: Include needed fields

   To get only customer and total, set them to 1 in projection.

2. Step 2: Exclude _id field

   Set _id to 0 to exclude it from results.

3. Step 3: Avoid mixing inclusion and exclusion

   All fields except _id must be included (1), no mixing with 0.

4. Final Answer:

   db.orders.find({}, {customer: 1, total: 1, _id: 0}) -> Option C

5. Quick Check:

   Include fields with 1, exclude _id with 0 [OK]

Hint: Include needed fields with 1, exclude _id with 0 [OK]

Common Mistakes:

• Including _id by default
• Mixing 1 and 0 in fields other than _id
• Excluding needed fields accidentally