Projection for selecting fields in MongoDB - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to select only the 'name' field from the 'users' collection.

MongoDB

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

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

Aemail

Bage

Cname

Daddress

Attempts:

3 left

💡 Hint

Common Mistakes

Using 0 instead of 1 to include a field.

Selecting a field not present in the collection.

To select only the 'name' field, you specify it with a value of 1 in the projection document.

2fill in blank

medium

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

MongoDB

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

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

Apassword

Bname

Cemail

Dage

Attempts:

3 left

💡 Hint

Common Mistakes

Using 1 instead of 0 to exclude a field.

Trying to exclude the _id field without specifying it.

Setting a field to 0 in the projection excludes it from the results.

3fill in blank

hard

Fix the error in the code to select only 'name' and 'email' fields.

MongoDB

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

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

Aemail

Baddress

Cpassword

Dage

Attempts:

3 left

💡 Hint

Common Mistakes

Using 0 instead of 1 for the second field.

Including fields not present in the collection.

To select multiple fields, list each with a value of 1 in the projection document.

4fill in blank

hard

Fill both blanks to exclude 'password' and 'ssn' fields from the results.

MongoDB

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

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

Apassword

Bemail

Cssn

Dname

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing 1 and 0 in the same projection document.

Forgetting to exclude all sensitive fields.

To exclude multiple fields, list each with 0 in the projection document.

5fill in blank

hard

Fill all three blanks to select 'name' and 'email' fields but exclude '_id' field.

MongoDB

db.users.find({}, { [1]: 1, [2]: 1, [3]: 0 })

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

Aname

Bemail

C_id

Dpassword

Attempts:

3 left

💡 Hint

Common Mistakes

Trying to include and exclude fields at the same time incorrectly.

Not excluding '_id' when only specific fields are selected.

To include fields, set them to 1; to exclude '_id', set it to 0.

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