Date expressions ($year, $month, $dayOfMonth) in MongoDB - Mini Project: Build & Apply

Extract Year, Month, and Day from Dates in MongoDB

📖 Scenario: You work at a small online store. The store keeps records of orders in a MongoDB collection called orders. Each order has a date field that stores when the order was placed.You want to create a new collection that shows the year, month, and day of each order separately. This helps the store analyze sales by date parts.

🎯 Goal: Create a MongoDB aggregation pipeline that extracts the year, month, and dayOfMonth from the date field of each order document.Store the results in a new collection called order_dates.

📋 What You'll Learn

Create an orders collection with three documents, each having a date field with exact ISODate values.

Define a variable pipeline that holds the aggregation pipeline array.

Use $year, $month, and $dayOfMonth expressions in the $project stage to extract date parts.

Use db.orders.aggregate(pipeline) to run the pipeline and db.order_dates.insertMany() to save the results.

💡 Why This Matters

🌍 Real World

Extracting parts of dates is common in sales, event tracking, and reporting systems to analyze data by year, month, or day.

💼 Career

Database developers and analysts often write aggregation pipelines to transform and summarize date data for business insights.

1

Create the orders collection with date data

Create a collection called orders with exactly these three documents, each having a date field with the given ISODate values:
{ _id: 1, date: ISODate("2023-01-15T10:00:00Z") },
{ _id: 2, date: ISODate("2023-06-20T15:30:00Z") },
{ _id: 3, date: ISODate("2023-12-05T08:45:00Z") }

MongoDB

// Insert the three documents into the orders collection
// Your code here

Hint

Use db.orders.insertMany() with an array of objects. Each object must have _id and date fields.

2

Define the aggregation pipeline variable

Create a variable called pipeline and set it to an array with one $project stage. For now, leave the $project stage empty (an empty object {}).

MongoDB

db.orders.insertMany([
  { _id: 1, date: ISODate("2023-01-15T10:00:00Z") },
  { _id: 2, date: ISODate("2023-06-20T15:30:00Z") },
  { _id: 3, date: ISODate("2023-12-05T08:45:00Z") }
])

// Define the pipeline variable with one $project stage
const pipeline = [
  { $project: { } }
]

Hint

Use const pipeline = [ { $project: { } } ] to create the pipeline variable.

3

Extract year, month, and day using date expressions

Update the $project stage inside the pipeline variable to include these fields:
- year using $year on $date
- month using $month on $date
- day using $dayOfMonth on $date

MongoDB

db.orders.insertMany([
  { _id: 1, date: ISODate("2023-01-15T10:00:00Z") },
  { _id: 2, date: ISODate("2023-06-20T15:30:00Z") },
  { _id: 3, date: ISODate("2023-12-05T08:45:00Z") }
])

const pipeline = [
  { $project: {
      // Extract year, month, and day from date
  } }
]

Hint

Use { year: { $year: "$date" }, month: { $month: "$date" }, day: { $dayOfMonth: "$date" } } inside $project.

4

Run the aggregation and save results to order_dates

Run the aggregation pipeline on orders using db.orders.aggregate(pipeline).toArray() and insert the results into a new collection called order_dates using db.order_dates.insertMany().

MongoDB

db.orders.insertMany([
  { _id: 1, date: ISODate("2023-01-15T10:00:00Z") },
  { _id: 2, date: ISODate("2023-06-20T15:30:00Z") },
  { _id: 3, date: ISODate("2023-12-05T08:45:00Z") }
])

const pipeline = [
  { $project: {
      year: { $year: "$date" },
      month: { $month: "$date" },
      day: { $dayOfMonth: "$date" }
  } }
]

// Run the aggregation and insert results into order_dates
// Your code here

Hint

Use const results = db.orders.aggregate(pipeline).toArray() and then db.order_dates.insertMany(results).

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