Array expressions ($size, $arrayElemAt, $filter) in MongoDB

Introduction

Array expressions help you work with lists inside your data. They let you count items, pick specific items, or find items that match a rule.

You want to count how many items are in a list inside your data.

You need to get a specific item from a list by its position.

You want to find only the items in a list that meet a certain condition.

Syntax

MongoDB

db.collection.aggregate([
  {
    $project: {
      totalItems: { $size: "$arrayField" },
      firstItem: { $arrayElemAt: ["$arrayField", 0] },
      filteredItems: {
        $filter: {
          input: "$arrayField",
          as: "item",
          cond: { /* <condition using $$item> */ }
        }
      }
    }
  }
])

$size counts how many elements are in an array.

$arrayElemAt gets the element at the given index (starting at 0).

$filter returns a new array with only elements that match the condition.

Examples

This counts how many tags each product has.

MongoDB

db.products.aggregate([
  { $project: { numberOfTags: { $size: "$tags" } } }
])

This gets the first tag from the tags array.

MongoDB

db.products.aggregate([
  { $project: { firstTag: { $arrayElemAt: ["$tags", 0] } } }
])

This finds tags that are exactly "expensive".

MongoDB

db.products.aggregate([
  {
    $project: {
      expensiveTags: {
        $filter: {
          input: "$tags",
          as: "tag",
          cond: { $eq: ["$$tag", "expensive"] }
        }
      }
    }
  }
])

Sample Program

This example inserts three orders with different items and prices. Then it counts how many items each order has, gets the first item, and filters prices greater than 1.

MongoDB

db.orders.insertMany([
  { _id: 1, items: ["apple", "banana", "orange"], prices: [1, 2, 3] },
  { _id: 2, items: ["bread"], prices: [2] },
  { _id: 3, items: [], prices: [] }
])

db.orders.aggregate([
  {
    $project: {
      totalItems: { $size: "$items" },
      firstItem: { $arrayElemAt: ["$items", 0] },
      expensivePrices: {
        $filter: {
          input: "$prices",
          as: "price",
          cond: { $gt: ["$$price", 1] }
        }
      }
    }
  }
])

Important Notes

$size returns 0 if the array is empty or missing.

$arrayElemAt returns null if the index is out of range.

$filter lets you use variables with $$ to refer to each item.

Counting items is fast, but filtering large arrays can be slower.

Summary

$size counts how many elements are in an array.

$arrayElemAt picks one element by its position.

$filter creates a smaller array with only items that match a rule.

Practice

1. What does the MongoDB expression $size do when used on an array field?

easy

A. Counts how many elements are in the array

B. Returns the first element of the array

C. Filters elements based on a condition

D. Sorts the array elements

Solution

1. Step 1: Understand the purpose of $size

   The $size operator counts the number of elements inside an array.

2. Step 2: Compare with other operators

   $arrayElemAt picks an element by position, $filter selects elements by condition, and sorting is unrelated.

3. Final Answer:

   Counts how many elements are in the array -> Option A

4. Quick Check:

   $size = count elements [OK]

Hint: Remember $size counts array length, not elements themselves [OK]

Common Mistakes:

• Confusing $size with $arrayElemAt
• Thinking $size filters elements
• Assuming $size sorts arrays

2. Which of the following is the correct syntax to get the third element of an array field named items using MongoDB aggregation?

easy

A. { $arrayElemAt: ["items", 3] }

B. { $size: ["$items", 2] }

C. { $arrayElemAt: ["$items", 2] }

D. { $filter: { input: "$items", as: "item", cond: { $eq: ["$$item", 2] } } }

Solution

1. Step 1: Recall zero-based indexing in $arrayElemAt

   The third element is at index 2 because counting starts at 0.

2. Step 2: Check syntax correctness

   The correct syntax uses $arrayElemAt with the array field prefixed by $ and the zero-based index.

3. Final Answer:

   { $arrayElemAt: ["$items", 2] } -> Option C

4. Quick Check:

   Third element index = 2 [OK]

Hint: Remember arrays start at 0, so third element is index 2 [OK]

Common Mistakes:

• Using 1-based index instead of 0-based
• Missing $ before field name
• Confusing $size with $arrayElemAt

3. Given the document { scores: [10, 20, 30, 40] }, what will the aggregation expression { $size: "$scores" } return?

medium

A. 4

B. 30

C. [10, 20, 30, 40]

D. Error

Solution

1. Step 1: Understand what $size does

   $size counts the number of elements in the array.

2. Step 2: Count elements in the array

   The array [10, 20, 30, 40] has 4 elements.

3. Final Answer:

   4 -> Option A

4. Quick Check:

   Array length = 4 [OK]

Hint: Count elements inside brackets for $size result [OK]

Common Mistakes:

• Returning the third element instead of count
• Returning the whole array instead of count
• Assuming syntax error

4. Identify the error in this MongoDB aggregation expression: { $arrayElemAt: ["$values", "1"] }

medium

A. Field name missing $ prefix

B. Index must be a number, not a string

C. Array must be inside an object

D. No error, expression is correct

Solution

1. Step 1: Check the type of index in $arrayElemAt

   The index must be a number, but here it is a string "1".

2. Step 2: Confirm correct field referencing

   The field name "$values" is correctly prefixed with $.

3. Final Answer:

   Index must be a number, not a string -> Option B

4. Quick Check:

   Index type = number, not string [OK]

Hint: Use number type for index in $arrayElemAt [OK]

Common Mistakes:

• Using string instead of number for index
• Omitting $ before field name
• Thinking array must be wrapped differently

5. You have documents with a field tags as an array of strings. How would you write a MongoDB aggregation expression to get the count of tags that start with the letter 'a'?

hard

A. { $size: "$tags" }

B. { $arrayElemAt: ["$tags", 0] }

C. { $filter: { input: "$tags", as: "tag", cond: { $eq: ["$$tag", "a"] } } }

D. { $size: { $filter: { input: "$tags", as: "tag", cond: { $regexMatch: { input: "$$tag", regex: /^a/ } } } } }

Solution

1. Step 1: Use $filter to select tags starting with 'a'

   The $filter operator creates a new array with tags matching the regex /^a/.

2. Step 2: Use $size to count filtered tags

   $size counts how many tags matched the condition.

3. Final Answer:

   { $size: { $filter: { input: "$tags", as: "tag", cond: { $regexMatch: { input: "$$tag", regex: /^a/ } } } } } -> Option D

4. Quick Check:

   Filter then count = correct approach [OK]

Hint: Filter first, then count with $size for conditional counts [OK]

Common Mistakes:

• Using $arrayElemAt instead of $filter
• Filtering with exact match instead of regex
• Counting whole array without filtering