$project stage for shaping output in MongoDB - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to include only the "name" field in the output documents.

MongoDB

db.collection.aggregate([{ $project: { [1]: 1 } }])

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

Aname

Baddress

Cage

Demail

Attempts:

3 left

💡 Hint

Common Mistakes

Using 0 instead of 1 to include a field.

Including multiple fields when only one is asked.

Using the wrong field name.

The $project stage shapes the output by including only the specified fields. Here, including name with value 1 means only the name field will appear in the output.

2fill in blank

medium

Complete the code to exclude the "password" field from the output documents.

MongoDB

db.collection.aggregate([{ $project: { [1]: 0 } }])

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

Apassword

Busername

Cemail

Dage

Attempts:

3 left

💡 Hint

Common Mistakes

Using 1 instead of 0 to exclude a field.

Excluding the wrong field.

Mixing inclusion and exclusion in the same $project stage.

Setting a field to 0 in $project excludes it from the output. Here, excluding password hides it from the results.

3fill in blank

hard

Fix the error in the $project stage to rename the "firstName" field to "name" in the output.

MongoDB

db.collection.aggregate([{ $project: { name: "$[1]" } }])

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

AlastName

BfirstName

CfullName

Dname

Attempts:

3 left

💡 Hint

Common Mistakes

Omitting the $ sign before the original field name.

Using the new field name instead of the original after the $ sign.

Not using quotes around the original field name.

To rename a field in $project, use the syntax newField: "$oldField". Here, name: "$firstName" renames firstName to name.

4fill in blank

hard

Fill both blanks to include "name" and exclude "_id" in the output documents.

MongoDB

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

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

Aname

B_id

Cage

Demail

Attempts:

3 left

💡 Hint

Common Mistakes

Trying to include and exclude the same field.

Forgetting to exclude _id when including other fields.

Using wrong field names.

Including name: 1 shows the name field, and excluding _id: 0 hides the default MongoDB ID field from output.

5fill in blank

hard

Fill all three blanks to rename "firstName" to "name", include "age", and exclude "_id" in the output.

MongoDB

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

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

Aname

Bage

C_id

Demail

Attempts:

3 left

💡 Hint

Common Mistakes

Not using $ before original field name when renaming.

Mixing inclusion and exclusion incorrectly.

Forgetting to exclude _id.

This $project stage renames firstName to name, includes the age field, and excludes the _id field from the output.

Practice

1. What does the $project stage do in a MongoDB aggregation pipeline?

easy

A. It filters documents based on conditions.

B. It shapes the output by selecting, renaming, or creating fields.

C. It sorts the documents in ascending or descending order.

D. It groups documents by a specified key.

Solution

1. Step 1: Understand the purpose of $project

   The $project stage is used to control which fields appear in the output documents and how they are shaped.

2. Step 2: Compare with other stages

   Filtering is done by $match, sorting by $sort, and grouping by $group. So $project is for shaping output fields.

3. Final Answer:

   It shapes the output by selecting, renaming, or creating fields. -> Option B

4. Quick Check:

   $project shapes output = A [OK]

Hint: Remember: $project controls output fields only [OK]

Common Mistakes:

• Confusing $project with $match for filtering
• Thinking $project sorts documents
• Mixing $project with $group for aggregation

2. Which of the following is the correct syntax to include only the name and age fields in the output using $project?

easy

A. { $project: { name: 1, age: 1 } }

B. { $project: { name: 0, age: 0 } }

C. { $project: { name: true, age: true } }

D. { $project: { name: 'include', age: 'include' } }

Solution

1. Step 1: Recall $project syntax for inclusion

   To include fields, set them to 1. To exclude, set to 0. Boolean or strings are invalid.

2. Step 2: Check each option

   { $project: { name: 1, age: 1 } } uses 1 for inclusion correctly. { $project: { name: 0, age: 0 } } excludes fields. { $project: { name: true, age: true } } uses true which is invalid. { $project: { name: 'include', age: 'include' } } uses strings which is invalid.

3. Final Answer:

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

4. Quick Check:

   Include fields = 1 in $project [OK]

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

Common Mistakes:

• Using true/false instead of 1/0
• Mixing inclusion and exclusion in same $project
• Using strings instead of numbers for field selection

3. Given the collection documents:

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

What will be the output of this aggregation?

[{ $project: { fullName: "$name", ageInMonths: { $multiply: ["$age", 12] }, _id: 0 } }]

medium

A. [{ "fullName": "Alice", "ageInMonths": 25 }]

B. [{ "name": "Alice", "ageInMonths": 300 }]

C. [{ "fullName": "Alice", "age": 25 }]

D. [{ "fullName": "Alice", "ageInMonths": 300 }]

Solution

1. Step 1: Understand field renaming and calculation

   The fullName field is created by copying name. The ageInMonths is calculated by multiplying age by 12 (25*12=300). The _id is excluded.

2. Step 2: Check output fields

   Output has fullName with "Alice" and ageInMonths with 300. No name or age fields remain.

3. Final Answer:

   [{ "fullName": "Alice", "ageInMonths": 300 }] -> Option D

4. Quick Check:

   Rename + multiply age = output [OK]

Hint: Use $multiply to calculate new fields in $project [OK]

Common Mistakes:

• Expecting original fields to remain without including them
• Confusing field names after renaming
• Forgetting to exclude _id if not wanted

4. Identify the error in this $project stage:

{ $project: { fullName: "$name", ageInMonths: { $multiply: ["age", 12] }, _id: 0 } }

medium

A. The _id field cannot be excluded.

B. Cannot use $multiply inside $project.

C. Field name "age" should be prefixed with $ to reference the field.

D. The field fullName cannot be renamed.

Solution

1. Step 1: Check field references in expressions

   In MongoDB aggregation, to reference a field inside expressions, prefix with $. Here, "age" is missing $.

2. Step 2: Validate other parts

   $multiply is valid in $project. Excluding _id is allowed. Renaming fields is allowed.

3. Final Answer:

   Field name "age" should be prefixed with $ to reference the field. -> Option C

4. Quick Check:

   Field references need $ prefix [OK]

Hint: Always prefix field names with $ inside expressions [OK]

Common Mistakes:

• Omitting $ before field names in expressions
• Thinking $multiply is invalid in $project
• Believing _id cannot be excluded

5. You have documents with fields firstName, lastName, and birthYear. Write a $project stage that outputs fullName as concatenation of first and last names separated by a space, and age calculated as 2024 minus birthYear. Which is the correct stage?

hard

A. { $project: { fullName: { $concat: ["$firstName", " ", "$lastName"] }, age: { $subtract: [2024, "$birthYear"] }, _id: 0 } }

B. { $project: { fullName: "$firstName $lastName", age: 2024 - "$birthYear", _id: 0 } }

C. { $project: { fullName: { $concat: ["firstName", " ", "lastName"] }, age: { $subtract: [2024, "birthYear"] }, _id: 0 } }

D. { $project: { fullName: { $concat: ["$firstName", "$lastName"] }, age: { $subtract: ["birthYear", 2024] }, _id: 0 } }

Solution

1. Step 1: Build fullName using $concat with spaces

   Use $concat with "$firstName", a space string, and "$lastName" to join names with a space.

2. Step 2: Calculate age using $subtract

   Subtract birthYear field from 2024 using $subtract. Field references need $.

3. Step 3: Check each option

   { $project: { fullName: { $concat: ["$firstName", " ", "$lastName"] }, age: { $subtract: [2024, "$birthYear"] }, _id: 0 } } correctly uses $concat with space and $subtract with field references. { $project: { fullName: "$firstName $lastName", age: 2024 - "$birthYear", _id: 0 } } uses invalid string concatenation and arithmetic. { $project: { fullName: { $concat: ["firstName", " ", "lastName"] }, age: { $subtract: [2024, "birthYear"] }, _id: 0 } } misses $ on fields. { $project: { fullName: { $concat: ["$firstName", "$lastName"] }, age: { $subtract: ["birthYear", 2024] }, _id: 0 } } misses space and reverses subtraction.

4. Final Answer:

   { $project: { fullName: { $concat: ["$firstName", " ", "$lastName"] }, age: { $subtract: [2024, "$birthYear"] }, _id: 0 } } -> Option A

5. Quick Check:

   Use $concat with space and $subtract with $field [OK]

Hint: Use $concat with spaces and $subtract with $field for calculations [OK]

Common Mistakes:

• Forgetting $ before field names
• Concatenating fields without spaces
• Subtracting in wrong order
• Using string expressions instead of operators