$addFields for computed fields in MongoDB - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to add a new field 'totalPrice' by multiplying 'price' and 'quantity'.

MongoDB

db.orders.aggregate([{ $addFields: { totalPrice: { $multiply: ["$price", [1]] } } }])

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

A"amount"

B"quantity"

C"$total"

D"$quantity"

Attempts:

3 left

💡 Hint

Common Mistakes

Forgetting the '$' prefix before field names.

Using the wrong field name without '$'.

In MongoDB aggregation, to refer to a field, prefix it with '$'. So to multiply 'price' and 'quantity', use "$quantity".

2fill in blank

medium

Complete the code to add a field 'fullName' by concatenating 'firstName' and 'lastName' with a space.

MongoDB

db.users.aggregate([{ $addFields: { fullName: { $concat: ["$firstName", [1], "$lastName"] } } }])

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

A" "

B"-"

C","

D"_"

Attempts:

3 left

💡 Hint

Common Mistakes

Using a dash or underscore instead of a space.

Forgetting to put the separator inside quotes.

To join first and last names with a space, use a space string " " inside $concat.

3fill in blank

hard

Fix the error in the code to add a field 'discountedPrice' by subtracting 'discount' from 'price'.

MongoDB

db.sales.aggregate([{ $addFields: { discountedPrice: { $subtract: ["price", [1]] } } }])

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

A"$discount"

B"discount"

C"$price"

D"amount"

Attempts:

3 left

💡 Hint

Common Mistakes

Omitting the '$' prefix before field names.

Using the wrong field name.

Field names in aggregation expressions must be prefixed with '$'. So use "$discount" to refer to the discount field.

4fill in blank

hard

Fill both blanks to add a field 'averageScore' by dividing 'totalScore' by 'numTests'.

MongoDB

db.students.aggregate([{ $addFields: { averageScore: { $divide: ["[1]", "[2]"] } } }])

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

A$totalScore

B$numTests

CtotalScore

DnumTests

Attempts:

3 left

💡 Hint

Common Mistakes

Using field names without '$' prefix.

Swapping numerator and denominator.

Use "$totalScore" and "$numTests" to refer to the fields inside the $divide operator.

5fill in blank

hard

Fill all three blanks to add a field 'summary' that concatenates uppercase 'category', a colon, and 'description'.

MongoDB

db.products.aggregate([{ $addFields: { summary: { $concat: [[1], [2], [3]] } } }])

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

A{'$toUpper': '$category'}

B": "

C$description

D$category

Attempts:

3 left

💡 Hint

Common Mistakes

Not using $toUpper for uppercase conversion.

Forgetting quotes around the colon and space string.

Using wrong field names.

Use $toUpper to convert 'category' to uppercase, then add a colon and space string ": ", then add 'description'.

Practice

1. What is the primary purpose of the $addFields stage in a MongoDB aggregation pipeline?

easy

A. To add new fields or update existing fields with computed values in documents

B. To delete fields from documents permanently

C. To sort documents based on a field

D. To group documents by a specific field

Solution

1. Step 1: Understand the role of $addFields

   $addFields is used to add new fields or update existing fields with computed values inside documents during aggregation.

2. Step 2: Compare with other stages

   Deleting fields is done by $project, sorting by $sort, and grouping by $group. These are different from $addFields.

3. Final Answer:

   To add new fields or update existing fields with computed values in documents -> Option A

4. Quick Check:

   $addFields adds or updates fields [OK]

Hint: Remember: $addFields enriches documents with new computed fields [OK]

Common Mistakes:

• Confusing $addFields with $project which removes fields
• Thinking $addFields sorts or groups documents
• Assuming $addFields deletes fields

2. Which of the following is the correct syntax to add a new field named totalPrice that multiplies price and quantity using $addFields?

easy

A. { $addFields: { totalPrice: "$price" * "$quantity" } }

B. { $addFields: { totalPrice: "$price" + "$quantity" } }

C. { $addFields: { totalPrice: { $multiply: ["$price", "$quantity"] } } }

D. { $addFields: { totalPrice: { $add: ["$price", "$quantity"] } } }

Solution

1. Step 1: Identify correct operator for multiplication

   MongoDB uses $multiply with an array of fields to multiply values.

2. Step 2: Check syntax correctness

   { $addFields: { totalPrice: { $multiply: ["$price", "$quantity"] } } } correctly uses $multiply with array syntax and field references with $.

3. Final Answer:

   { $addFields: { totalPrice: { $multiply: ["$price", "$quantity"] } } } -> Option C

4. Quick Check:

   Use $multiply with array for multiplication [OK]

Hint: Use $multiply with array of fields for multiplication [OK]

Common Mistakes:

• Using arithmetic operators like * directly inside $addFields
• Confusing $add with multiplication
• Missing $ before field names

3. Given the collection documents:

{ "item": "apple", "price": 2, "quantity": 5 }

What will be the output document after applying this aggregation stage?

{ $addFields: { totalCost: { $multiply: ["$price", "$quantity"] } } }

medium

A. Error: Invalid operator

B. { "item": "apple", "price": 2, "quantity": 5, "totalCost": "2*5" }

C. { "item": "apple", "price": 2, "quantity": 5 }

D. { "item": "apple", "price": 2, "quantity": 5, "totalCost": 10 }

Solution

1. Step 1: Calculate the multiplication of price and quantity

   2 multiplied by 5 equals 10.

2. Step 2: Understand $addFields adds the computed field

   The new field totalCost will be added with value 10.

3. Final Answer:

   { "item": "apple", "price": 2, "quantity": 5, "totalCost": 10 } -> Option D

4. Quick Check:

   2 * 5 = 10 added as totalCost [OK]

Hint: Multiply fields inside $addFields to get computed values [OK]

Common Mistakes:

• Expecting string expression instead of computed number
• Forgetting to add the new field in output
• Assuming syntax error when operator is correct

4. Identify the error in this aggregation stage:

{ $addFields: { discountPrice: { $multiply: "$price", 0.9 } } }

medium

A. Cannot multiply a field by a number in $addFields

B. Incorrect use of $multiply operator syntax; expects an array

C. Field name discountPrice is invalid

D. Missing $ before price

Solution

1. Step 1: Check $multiply syntax

   $multiply requires an array of expressions, but here it is given two separate arguments.

2. Step 2: Identify correct syntax

   The correct syntax is { $multiply: ["$price", 0.9] } using an array.

3. Final Answer:

   Incorrect use of $multiply operator syntax; expects an array -> Option B

4. Quick Check:

   $multiply needs array of operands [OK]

Hint: Always use array syntax with $multiply [OK]

Common Mistakes:

• Passing arguments to $multiply without array brackets
• Assuming field names cannot be multiplied by numbers
• Confusing field name syntax with missing $

5. You have documents with fields price and taxRate. You want to add a field finalPrice that is price plus price multiplied by taxRate. Which $addFields stage correctly computes this?

hard

A. { $addFields: { finalPrice: { $add: ["$price", { $multiply: ["$price", "$taxRate"] }] } } }

B. { $addFields: { finalPrice: "$price" + "$price" * "$taxRate" } }

C. { $addFields: { finalPrice: { $multiply: ["$price", "$taxRate"] } } }

D. { $addFields: { finalPrice: { $add: ["$price", "$taxRate"] } } }

Solution

1. Step 1: Understand the formula for finalPrice

   The formula is price + (price * taxRate), which requires addition and multiplication.

2. Step 2: Check correct MongoDB operators usage

   { $addFields: { finalPrice: { $add: ["$price", { $multiply: ["$price", "$taxRate"] }] } } } uses $add with an array containing price and the multiplication result of price and taxRate, which is correct syntax.

3. Final Answer:

   { $addFields: { finalPrice: { $add: ["$price", { $multiply: ["$price", "$taxRate"] }] } } } -> Option A

4. Quick Check:

   Use nested $add and $multiply for combined calculations [OK]

Hint: Nest $multiply inside $add for combined calculations [OK]

Common Mistakes:

• Trying to use arithmetic operators directly inside $addFields
• Adding price and taxRate directly without multiplication
• Using wrong operator or missing array brackets