Projection for selecting fields in MongoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Projection to include specific fields

Given a MongoDB collection users with documents containing name, age, and email, which query will return only the name and email fields for all users?

MongoDB

db.users.find({}, {name: 1, email: 1, _id: 0})

A db.users.find({}, {name: 0, email: 0})

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

C db.users.find({}, {name: 1, email: 1, _id: 0})

D db.users.find({}, {name: 1, email: 0})

💡 Hint

Remember to exclude the _id field if you don't want it in the output.

Option C includes name and email fields and excludes _id. Option C includes _id by default. Options A and D exclude or mix fields incorrectly.

❓ query_result

intermediate

Projection excluding a field

Which MongoDB query will return all fields except the password field from the accounts collection?

MongoDB

db.accounts.find({}, {password: 0})

A db.accounts.find({}, {password: 0})

B db.accounts.find({}, {password: false})

C db.accounts.find({}, {password: 1})

D db.accounts.find({}, {password: null})

💡 Hint

Use 0 to exclude a field in projection.

Option A correctly excludes the password field. Option A includes only password. Options C and D use invalid values for projection.

📝 Syntax

advanced

Identify the syntax error in projection

Which option contains a syntax error in the MongoDB projection part of the query?

MongoDB

db.products.find({}, {name: 1, price: 'yes'})

A db.products.find({}, {name: 1, price: 'yes'})

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

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

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

💡 Hint

Projection values must be 0 or 1, not strings.

Option A uses a string 'yes' instead of 0 or 1, causing a syntax error. Other options use valid numeric values.

❓ query_result

advanced

Projection with nested fields

Given documents with a nested address field containing city and zip, which query returns only the name and address.city fields?

MongoDB

db.customers.find({}, {name: 1, 'address.city': 1, _id: 0})

A db.customers.find({}, {name: 1, address: 0})

B db.customers.find({}, {name: 1, address: 1})

C db.customers.find({}, {name: 1, 'address.zip': 1})

D db.customers.find({}, {name: 1, 'address.city': 1, _id: 0})

💡 Hint

Use dot notation to project nested fields.

Option D correctly projects name and nested address.city fields. Option D includes whole address. Option D projects wrong nested field. Option D excludes address.

🧠 Conceptual

expert

Understanding projection behavior with _id field

In MongoDB, when you specify a projection like {name: 1} without mentioning _id, what will be the output regarding the _id field?

A The <code>_id</code> field is excluded by default in the output.

B The <code>_id</code> field is included by default in the output.

C The query will throw an error because <code>_id</code> is not specified.

D The <code>_id</code> field will be included only if explicitly set to 1.

💡 Hint

Think about MongoDB's default behavior for _id in projections.

MongoDB includes the _id field by default unless explicitly excluded with _id: 0. So option B is correct.

Practice

1. What does projection do in a MongoDB query?

easy

A. It selects which fields to include or exclude in the query result.

B. It sorts the documents in the collection.

C. It updates the documents in the collection.

D. It deletes documents from the collection.

Solution

1. Step 1: Understand the purpose of projection

   Projection in MongoDB is used to specify which fields to include or exclude in the output of a query.

2. Step 2: Differentiate projection from other operations

   Sorting, updating, and deleting are different operations and not related to projection.

3. Final Answer:

   It selects which fields to include or exclude in the query result. -> Option A

4. Quick Check:

   Projection = Select fields [OK]

Hint: Projection picks fields to show, not to sort or update [OK]

Common Mistakes:

• Confusing projection with sorting
• Thinking projection updates data
• Assuming projection deletes documents

2. Which of the following is the correct syntax to include only the name and age fields in a MongoDB find query?

easy

A. { name: 1, age: 1 }

B. { name: true, age: true }

C. { name: 'include', age: 'include' }

D. { name: 0, age: 0 }

Solution

1. Step 1: Recall projection syntax

   In MongoDB, to include fields, you set them to 1 in the projection document.

2. Step 2: Check each option

   { name: 1, age: 1 } uses 1 correctly; { name: true, age: true } uses true which is valid in MongoDB; { name: 'include', age: 'include' } uses strings which is invalid; { name: 0, age: 0 } excludes fields instead of including.

3. Final Answer:

   { name: 1, age: 1 } -> Option A

4. Quick Check:

   Include fields = 1 [OK]

Hint: Use 1 to include fields, not strings [OK]

Common Mistakes:

• Using strings like 'include'
• Using 0 to include fields

3. Given the collection documents:

{ _id: 1, name: "Alice", age: 25, city: "NY" }

{ _id: 2, name: "Bob", age: 30, city: "LA" }

What will be the result of the query db.collection.find({}, { name: 1, city: 1 })?

medium

A. [{ _id: 1, age: 25 }, { _id: 2, age: 30 }]

B. [{ name: "Alice", city: "NY" }, { name: "Bob", city: "LA" }]

C. [{ _id: 1, name: "Alice", city: "NY" }, { _id: 2, name: "Bob", city: "LA" }]

D. [{ name: "Alice", age: 25, city: "NY" }, { name: "Bob", age: 30, city: "LA" }]

Solution

1. Step 1: Understand default _id behavior

   By default, MongoDB includes the _id field unless explicitly excluded.

2. Step 2: Check projection fields

   The query includes name and city fields with 1, so these fields plus _id will appear.

3. Final Answer:

   [{ _id: 1, name: "Alice", city: "NY" }, { _id: 2, name: "Bob", city: "LA" }] -> Option C

4. Quick Check:

   Projection includes fields + _id by default [OK]

Hint: Projection includes _id unless set to 0 [OK]

Common Mistakes:

• Assuming _id is excluded by default
• Expecting age field in result
• Confusing included and excluded fields

4. What is wrong with this MongoDB query to exclude the password field?

db.users.find({}, { password: 1 })

medium

A. The syntax for projection is incorrect.

B. It includes the password field instead of excluding it.

C. You cannot exclude fields in MongoDB projection.

D. The query will return an error because password is a reserved word.

Solution

1. Step 1: Understand projection values

   Setting a field to 1 includes it; setting to 0 excludes it.

2. Step 2: Analyze the query

   The query sets password: 1, so it includes the password field (and _id), not excludes it.

3. Final Answer:

   It includes the password field instead of excluding it. -> Option B

4. Quick Check:

   Use 0 to exclude fields [OK]

Hint: Use 0 to exclude fields, 1 to include [OK]

Common Mistakes:

• Using 1 to exclude fields
• Thinking password is reserved word
• Believing exclusion is not possible

5. You want to fetch documents from a collection but exclude the _id and password fields while including all others. Which projection is correct?

hard

A. { _id: 1, password: 1 }

B. { _id: 1, password: 0 }

C. { _id: 0, password: 1 }

D. { _id: 0, password: 0 }

Solution

1. Step 1: Understand exclusion in projection

   Setting fields to 0 excludes them from the result.

2. Step 2: Apply exclusion to _id and password

   To exclude both _id and password, set both to 0 in the projection.

3. Final Answer:

   { _id: 0, password: 0 } -> Option D

4. Quick Check:

   Exclude fields with 0, include others by default [OK]

Hint: Set unwanted fields to 0, others stay included [OK]

Common Mistakes:

• Mixing inclusion and exclusion in projection
• Forgetting to exclude _id explicitly
• Setting excluded fields to 1