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.