Date expressions ($year, $month, $dayOfMonth) in MongoDB - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to extract the year from the date field.

MongoDB

{ $project: { year: { $[1]: "$date" } } }

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

Ayear

Bmonth

CdayOfMonth

Dhour

Attempts:

3 left

💡 Hint

Common Mistakes

Using $month or $dayOfMonth instead of $year.

Forgetting the $ sign before the operator.

The $year operator extracts the year part from a date.

2fill in blank

medium

Complete the code to extract the month from the date field.

MongoDB

{ $project: { month: { $[1]: "$date" } } }

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

AdayOfMonth

Bmonth

Cyear

Dminute

Attempts:

3 left

💡 Hint

Common Mistakes

Using $dayOfMonth or $year instead of $month.

Confusing month with minute or hour.

The $month operator extracts the month number (1-12) from a date.

3fill in blank

hard

Fix the error in the code to extract the day of the month from the date field.

MongoDB

{ $project: { day: { $[1]: "$date" } } }

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

Adate

Bmonth

CdayOfMonth

Dday

Attempts:

3 left

💡 Hint

Common Mistakes

Using $day or $date which are not valid operators.

Using $month instead of $dayOfMonth.

The correct operator to get the day of the month is $dayOfMonth.

4fill in blank

hard

Fill both blanks to extract the year and month from the date field.

MongoDB

{ $project: { year: { $[1]: "$date" }, month: { $[2]: "$date" } } }

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

Ayear

BdayOfMonth

Cmonth

Dhour

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping year and month operators.

Using $dayOfMonth or $hour instead of month.

Use $year to get the year and $month to get the month from a date.

5fill in blank

hard

Fill all three blanks to extract year, month, and day of the month from the date field.

MongoDB

{ $project: { year: { $[1]: "$date" }, month: { $[2]: "$date" }, day: { $[3]: "$date" } } }

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

Ayear

Bmonth

CdayOfMonth

Dminute

Attempts:

3 left

💡 Hint

Common Mistakes

Using $day or $date instead of $dayOfMonth.

Mixing up month and dayOfMonth operators.

Use $year, $month, and $dayOfMonth to extract the respective parts from a date.

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