Date expressions ($year, $month, $dayOfMonth) in MongoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Extract the year from a date field

Given a MongoDB collection events with documents containing a date field, which aggregation pipeline stage correctly extracts the year from the date field into a new field called year?

MongoDB

db.events.aggregate([
  { $project: { year: ??? } }
])

A { year: { $year: "$date" } }

B { year: { $dayOfMonth: "$date" } }

C { year: { $month: "$date" } }

D { year: { $dateToString: { format: "%Y", date: "$date" } } }

💡 Hint

Use the $year operator to get the year part of a date.

The $year operator extracts the year from a date. $month and $dayOfMonth extract other parts, and $dateToString returns a string, not a number.

❓ query_result

intermediate

Find documents from a specific month

You want to find all documents in the orders collection where the orderDate is in March (month 3). Which aggregation pipeline stage filters documents correctly?

MongoDB

db.orders.aggregate([
  { $match: { ??? } }
])

A { $expr: { $eq: [ { $month: "$orderDate" }, 3 ] } }

B { orderDate: { $eq: 3 } }

C { $expr: { $eq: [ { $dayOfMonth: "$orderDate" }, 3 ] } }

D { orderDate: { $month: 3 } }

💡 Hint

Use $expr to compare the month part of the date.

Only option A uses $expr with $month to compare the month number correctly. Others are invalid or compare wrong parts.

📝 Syntax

advanced

Identify the syntax error in date extraction

Which option contains a syntax error when trying to extract the day of the month from a createdAt field in an aggregation pipeline?

MongoDB

db.users.aggregate([
  { $project: { day: ??? } }
])

A } } "tAdetaerc$" :htnoMfOyad$ { :yad {

B { day: { $dayOfMonth: ["$createdAt"] } }

C { day: { $dayOfMonth: "$createdAt" } }

D { day: { $dayOfMonth: createdAt } }

💡 Hint

Field names must be strings with $ prefix inside quotes.

Option D misses quotes around "$createdAt", causing a syntax error. Options A and C are correct syntax. Option D uses an array which is invalid for $dayOfMonth.

🧠 Conceptual

advanced

Understanding output types of date expressions

What is the data type of the value returned by the MongoDB $year, $month, and $dayOfMonth expressions when used in an aggregation pipeline?

A Date object representing the extracted part

B String representing the year, month, or day

C Integer number representing the year, month, or day

D Boolean indicating if the date part exists

💡 Hint

Think about what kind of value you get when you ask for a year or month number.

These expressions return integer numbers, for example, 2024 for year, 6 for June month, or 15 for day 15.

❓ query_result

expert

Combine $year, $month, and $dayOfMonth to format a date string

You want to create a new field formattedDate in the format YYYY-MM-DD using $year, $month, and $dayOfMonth from a timestamp field. Which aggregation pipeline stage produces the correct string format with leading zeros for month and day?

MongoDB

db.logs.aggregate([
  { $project: { formattedDate: ??? } }
])

A

{ $concat: [
  { $toString: { $year: "$timestamp" } }, "-",
  { $toString: { $month: "$timestamp" } }, "-",
  { $toString: { $dayOfMonth: "$timestamp" } }
] }

B

{ $concat: [
  { $toString: { $year: "$timestamp" } }, "-",
  { $cond: [ { $lt: [ { $month: "$timestamp" }, 10 ] }, { $concat: ["0", { $toString: { $month: "$timestamp" } }] }, { $toString: { $month: "$timestamp" } } ] }, "-",
  { $cond: [ { $lt: [ { $dayOfMonth: "$timestamp" }, 10 ] }, { $concat: ["0", { $toString: { $dayOfMonth: "$timestamp" } }] }, { $toString: { $dayOfMonth: "$timestamp" } } ] }
] }

C { $dateToString: { format: "%Y-%m-%d", date: "$timestamp" } }

D

{ $concat: [
  { $year: "$timestamp" }, "-",
  { $month: "$timestamp" }, "-",
  { $dayOfMonth: "$timestamp" }
] }

💡 Hint

You need to add leading zeros for month and day if less than 10.

Option B uses $cond to check if month or day is less than 10 and adds a leading zero, then concatenates all parts as strings. Option B misses leading zeros. Option B uses $dateToString which is simpler but does not use $year, $month, $dayOfMonth as requested. Option B tries to concat numbers directly causing error.

Practice

1. What does the MongoDB expression {$year: "$dateField"} return when used in an aggregation pipeline?

easy

A. The month number of the date

B. The year part of the date stored in dateField

C. The full date as a string

D. The day of the week of the date

Solution

1. Step 1: Understand the $year operator

   The $year expression extracts the year component from a date value.

2. Step 2: Apply to the field dateField

   Using {$year: "$dateField"} returns the year number from the date stored in dateField.

3. Final Answer:

   The year part of the date stored in dateField -> Option B

4. Quick Check:

   $year extracts year [OK]

Hint: Remember $year extracts only the year number from a date [OK]

Common Mistakes:

• Confusing $year with $month or $dayOfMonth
• Expecting a full date string instead of just the year
• Using $year outside aggregation pipeline

2. Which of the following is the correct syntax to extract the month from a date field named createdAt in a MongoDB aggregation pipeline?

easy

A. { $month: "$createdAt" }

B. { $month: createdAt }

C. { month: "$createdAt" }

D. { $month: "createdAt" }

Solution

1. Step 1: Identify correct field reference syntax

   In MongoDB aggregation, field names must be referenced as strings with a $ prefix, like "$createdAt".

2. Step 2: Confirm correct operator usage

   The operator to extract month is $month, so the correct syntax is { $month: "$createdAt" }.

3. Final Answer:

   { $month: "$createdAt" } -> Option A

4. Quick Check:

   Field names need "$" prefix in aggregation [OK]

Hint: Use "$fieldName" to reference fields inside aggregation [OK]

Common Mistakes:

• Omitting quotes and $ prefix around field name
• Using operator without $ sign
• Passing field name as plain string without $

3. Given the following aggregation stage, what will be the output for a document with dateField value of ISODate("2023-07-15T10:00:00Z")?

{ $project: { year: { $year: "$dateField" }, month: { $month: "$dateField" }, day: { $dayOfMonth: "$dateField" } } }

medium

A. { year: 2023, month: 7, day: 14 }

B. { year: 2023, month: 6, day: 15 }

C. { year: 2023, month: 7, day: 15 }

D. { year: 2022, month: 7, day: 15 }

Solution

1. Step 1: Extract year, month, and day from the date

   The date is July 15, 2023. So, year = 2023, month = 7 (July), day = 15.

2. Step 2: Confirm output matches extracted values

   The output document will have year: 2023, month: 7, day: 15 as projected.

3. Final Answer:

   { year: 2023, month: 7, day: 15 } -> Option C

4. Quick Check:

   Date parts match output [OK]

Hint: Remember months are 1-based in MongoDB date expressions [OK]

Common Mistakes:

• Assuming months start at 0 instead of 1
• Confusing dayOfMonth with dayOfWeek
• Misreading the date value

4. You wrote this aggregation stage to get the day of the month from orderDate:

{ $project: { day: { $dayOfMonth: orderDate } } }

But it throws an error. What is the mistake?

medium

A. Missing quotes and $ before field name in $dayOfMonth

B. Using $project instead of $match

C. Field orderDate is not a date type

D. Incorrect operator; should use $day instead

Solution

1. Step 1: Check field reference syntax inside $dayOfMonth

   Field names must be referenced as strings with a $ prefix, so it should be "$orderDate".

2. Step 2: Identify the cause of error

   Using orderDate without quotes and $ causes a syntax error in aggregation.

3. Final Answer:

   Missing quotes and $ before field name in $dayOfMonth -> Option A

4. Quick Check:

   Field names need "$" prefix in aggregation [OK]

Hint: Always use "$fieldName" inside aggregation expressions [OK]

Common Mistakes:

• Forgetting $ and quotes around field names
• Assuming $day is a valid operator
• Confusing aggregation stages

5. You want to group sales documents by the year and month of their saleDate field. Which aggregation pipeline stage correctly groups by year and month using date expressions?

hard

A. { $group: { _id: { year: { $year: "$saleDate" }, month: "$month" }, totalSales: { $sum: "$amount" } } }

B. { $group: { _id: { year: "$year", month: "$month" }, totalSales: { $sum: "$amount" } } }

C. { $group: { _id: { year: { $year: saleDate }, month: { $month: saleDate } }, totalSales: { $sum: "$amount" } } }

D. { $group: { _id: { year: { $year: "$saleDate" }, month: { $month: "$saleDate" } }, totalSales: { $sum: "$amount" } } }

Solution

1. Step 1: Use correct field references with $year and $month

   Field names must be referenced as strings with $ prefix, so "$saleDate" is correct.

2. Step 2: Verify grouping keys and sum operator

   Grouping by year and month extracted from saleDate and summing amount is done correctly in { $group: { _id: { year: { $year: "$saleDate" }, month: { $month: "$saleDate" } }, totalSales: { $sum: "$amount" } } }.

3. Final Answer:

   { $group: { _id: { year: { $year: "$saleDate" }, month: { $month: "$saleDate" } }, totalSales: { $sum: "$amount" } } } -> Option D

4. Quick Check:

   Correct field references and operators used [OK]

Hint: Group by {_id: {year: {$year: "$field"}, month: {$month: "$field"}}} [OK]

Common Mistakes:

• Using field names without $ and quotes
• Mixing string field names with expressions
• Incorrectly mixing aggregation operators