$addFields for computed fields in MongoDB - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the $addFields stage do in a MongoDB aggregation pipeline?

It adds new fields or modifies existing fields in the documents by computing values, without removing any existing fields.

beginner

How can you use $addFields to create a new field that is the sum of two existing fields, price and tax?

Use $addFields with an expression like: { totalCost: { $add: ["$price", "$tax"] } } to add a new field totalCost.

beginner

Can $addFields remove fields from documents?

No, $addFields only adds or updates fields. To remove fields, use $project or $unset.

intermediate

What happens if you use $addFields to add a field that already exists in the document?

The existing field is overwritten with the new computed value.

intermediate

Give an example of using $addFields to add a field isAdult that is true if age is 18 or more, false otherwise.

Use: { $addFields: { isAdult: { $gte: ["$age", 18] } } }. This adds isAdult as true or false based on age.

What is the main purpose of the $addFields stage in MongoDB?

A Sort documents

B Add or update fields in documents

C Remove fields from documents

D Group documents

$addFields adds new fields or updates existing ones without removing any fields.

Which operator can you use inside $addFields to sum two fields price and tax?

A $multiply

B $sum

C $add

D $concat

$add is used to add numeric values inside $addFields.

If you want to remove a field from documents, which stage should you use instead of $addFields?

A $unset

B $addFields

C $group

D $match

$unset removes fields from documents.

What happens if $addFields adds a field that already exists?

A The existing field is overwritten

B The field is ignored

C The field is duplicated

D It causes an error

The existing field is replaced with the new computed value.

How would you add a boolean field isAdult that is true if age is 18 or more?

A { isAdult: { $lt: ["$age", 18] } }

B { isAdult: { $ne: ["$age", 18] } }

C { isAdult: { $eq: ["$age", 18] } }

D { isAdult: { $gte: ["$age", 18] } }

$gte checks if age is greater than or equal to 18.

Explain how $addFields can be used to compute new fields in MongoDB documents.

Think about how you can add new information to each document without losing old data.

Describe a scenario where $addFields is useful in a data aggregation pipeline.

Consider when you want to enrich your data with new calculated details.

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