Exercise 4 — Expressions Deep Dive

Topics: Arithmetic, comparison, boolean, control flow ($cond, $switch), and array operators ($map, $filter, $reduce)

Reading before you start: 08 - Expressions Overview, 09 - Arithmetic Operators, 10 - Comparison Operators, 11 - Boolean and Control Flow Operators, 12 - Array Operators

Database: aggregation_lab — see Setup - Database

Part A — Arithmetic and Comparison Expressions

Task A1

For each project, compute a field budgetUtilization as (actualCost / estimatedCost) * 100. Also compute budgetRemaining as estimatedCost - actualCost. Project title, budgetUtilization, and budgetRemaining.

Hint: Use $divide and $multiply nested together for the percentage.

Task A2

For each employee, compute yearsOfExperience as (2025 - year_of_hire). Since we can’t easily extract the year from a date, approximate by computing salary / 5000 and call it salaryTier instead. Also compute doubledSalary as salary * 2.

Task A3

For each order, unwind items and compute lineTotal as price * quantity. Then compute discountedTotal as lineTotal * 0.9 (10% discount). Project product, lineTotal, and discountedTotal.

Task A4

For each task, add a field isHeavy using $gt to check if hours > 15. Also add cmpTo10 using $cmp to compare hours with 10.

Project title, hours, isHeavy, and cmpTo10.

Part B — Boolean and Control Flow

Task B1

For each employee, use $cond to add a field salaryCategory:

If salary >= 60000 → "SENIOR"
Otherwise → "STANDARD"

Project name, salary, and salaryCategory.

Task B2

For each project, use $cond to add a field overBudget:

If actualCost > estimatedCost → true
Otherwise → false

Task B3

For each employee, use $switch to add a field experienceLevel:

If role is "INTERN" → "JUNIOR"
If role is "DEVELOPER" → "MID"
If role is "LEAD" or "MANAGER" → "SENIOR"
Default → "OTHER"

Hint: For the LEAD/MANAGER case, you can use $in inside $switch to check if the role is in a list, or you can have two separate case branches.

Task B4

For each task, use $cond with $and to add a field urgent:

If the task is "TODO" and hours >= 15 → true
Otherwise → false

Task B5

For each order, use $switch to add a field statusLabel:

"DELIVERED" → "Completed"
"SHIPPED" → "On the way"
"PROCESSING" → "Being prepared"
"CANCELLED" → "Cancelled"
Default → "Unknown"

Part C — Array Operators

Task C1

Use $filter on the employees collection to add a field highRatings that contains only ratings greater than or equal to 4 from the ratings array.

Project name and highRatings.

Task C2

Use $map on the employees collection to add a field ratingsPlusOne where each rating in the ratings array is increased by 1.

Project name, ratings (original), and ratingsPlusOne.

Task C3

Use $filter on the projects collection to add a field completedMilestones that only includes milestones where completed is true.

Then add a field completedCount using $size on completedMilestones.

Project title, completedCount, and completedMilestones.

Task C4

Use $reduce on the employees collection to add a field skillString that concatenates all skills into a single comma-separated string.

Hint:

$reduce: {
  input: "$skills",
  initialValue: "",
  in: {
    $cond: [
      { $eq: ["$$value", ""] },
      "$$this",
      { $concat: ["$$value", ", ", "$$this"] }
    ]
  }
}

Project name and skillString.

Task C5

For each order, use $map to transform the items array so that each item gets a new field lineTotal (price * quantity):

$map: {
  input: "$items",
  as: "item",
  in: {
    product: "$$item.product",
    quantity: "$$item.quantity",
    price: "$$item.price",
    lineTotal: { $multiply: ["$$item.price", "$$item.quantity"] }
  }
}

Project _id, date, and the transformed items.

Part D — Combining Expressions with Stages

Task D1

Find all employees where the average of their ratings is above 4.0.

Add a field avgRating using $avg on ratings
Match where avgRating > 4 (you’ll need $expr with $gt — or just use $match with a regular comparison on the computed field)
Project name, avgRating
Sort by avgRating descending

Task D2

For each project:

Add teamSize, milestoneCount, completedMilestones (filtered array), completedCount
Add completionRate computed as (completedCount / milestoneCount) * 100
Use $cond to add onTrack: true if completionRate >= 50, else false
Project title, status, teamSize, completionRate, onTrack
Sort by completionRate descending

Task D3

For each order:

Use $map to add lineTotal to each item in the items array
Use $addFields to add orderTotal as the $sum of all items.lineTotal values after the map — or use $reduce to sum them
Use $cond to add orderSize: "LARGE" if orderTotal > 500, else "SMALL"
Project date, status, orderTotal, orderSize
Sort by orderTotal descending

Part E — Creative Challenge

Task E1

Employee scoring system: Design a pipeline that computes a “performance score” for each employee using any combination of expressions. Your score formula might consider: average rating, number of skills, salary relative to department average, etc. Be creative and explain your formula in a comment.

Task E2

Smart log analysis: Using the log_entries collection, write a pipeline that:

Adds a field severity using $switch based on level (ERROR=3, WARNING=2, INFO=1)
Filters to only entries with severity >= 2
Groups by service, computing totalSeverity and entryCount
Sorts by totalSeverity descending

This mimics a real monitoring dashboard that highlights the most problematic services.

Task E3

Order intelligence: Create a pipeline on orders that transforms each order into a “receipt” format:

Each item should have its lineTotal computed (use $map)
The order should have orderTotal (sum of line totals)
Add itemCount (number of items)
Use $cond to add freeShipping: true if orderTotal > 1000
Project everything cleanly and save to a collection order_receipts

Reflection:

How does $map differ from $unwind? When would you prefer one over the other?

Why is $filter more efficient than $unwind + $match for filtering array elements?

What are $$value and $$this in $reduce, and how do they change as the reduction progresses?

Deep Thought

Explorer

Exercise 4 - Expressions