$slice modifier with $push in MongoDB - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to add a value to the 'scores' array using $push.

MongoDB

db.students.updateOne({name: 'Alice'}, {$push: {scores: [1])

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

A95

B[95]

C{95}

D"95"

Attempts:

3 left

💡 Hint

Common Mistakes

Using brackets [] which adds an array instead of a single value.

The $push operator adds a single value to an array. Here, 95 is the value to add.

2fill in blank

medium

Complete the code to limit the 'scores' array to the last 3 elements after pushing a new score.

MongoDB

db.students.updateOne({name: 'Bob'}, {$push: {scores: {$each: [88], [1]: -3}}})

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

A$count

B$limit

C$max

D$slice

Attempts:

3 left

💡 Hint

Common Mistakes

Using $limit which is not valid in this context.

The $slice modifier limits the array size after pushing. Using -3 keeps the last 3 elements.

3fill in blank

hard

Fix the error in the code to correctly push multiple scores and keep only the last 5.

MongoDB

db.students.updateOne({name: 'Carol'}, {$push: {scores: {$each: [1], $slice: -5}}})

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

A"70, 75, 80"

B{70, 75, 80}

C[70, 75, 80]

D70, 75, 80

Attempts:

3 left

💡 Hint

Common Mistakes

Using curly braces {} which is for objects, not arrays.

$each expects an array of values. Use square brackets [] to provide multiple scores.

4fill in blank

hard

Fill both blanks to push scores and keep only the first 4 elements in the array.

MongoDB

db.students.updateOne({name: 'Dave'}, {$push: {scores: {$each: [1], [2]: 4}}})

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

A[85, 90]

B$slice

C$limit

D$count

Attempts:

3 left

💡 Hint

Common Mistakes

Using $limit which is not a valid modifier here.

Use $each with an array to push multiple values, and $slice with a positive number to keep the first elements.

5fill in blank

hard

Fill all three blanks to push a score, keep last 3 elements, and sort scores descending.

MongoDB

db.students.updateOne({name: 'Eve'}, {$push: {scores: {$each: [[1]], [2]: -3, [3]: -1}}})

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

A92

B$slice

C$sort

D$limit

Attempts:

3 left

💡 Hint

Common Mistakes

Using $limit which is not a valid modifier here.

Push score 92, use $slice to keep last 3 elements, and $sort with -1 to sort descending.

Practice

1. What does the $slice modifier do when used with $push in MongoDB?

easy

A. Duplicates the array elements before pushing new ones

B. Removes all elements from the array before adding new ones

C. Limits the size of the array after adding new elements

D. Sorts the array elements in ascending order

Solution

1. Step 1: Understand $push behavior

   $push adds new elements to an array field in a document.

2. Step 2: Role of $slice modifier

   $slice limits the array size after pushing, keeping only a specified number of elements.

3. Final Answer:

   Limits the size of the array after adding new elements -> Option C

4. Quick Check:

   $slice limits array size [OK]

Hint: Remember: $slice trims array size after $push [OK]

Common Mistakes:

• Thinking $slice removes elements before pushing
• Confusing $slice with sorting or duplicating
• Assuming $slice adds elements instead of limiting

2. Which of the following is the correct syntax to push multiple items and limit the array size to 3 using $slice in MongoDB?

easy

A. { $push: { scores: { $each: [90, 85], $slice: 3 } } }

B. { $push: { scores: { $slice: [3], $each: [90, 85] } } }

C. { $push: { scores: { $each: 3, $slice: [90, 85] } } }

D. { $push: { scores: { $slice: [90, 85], $each: 3 } } }

Solution

1. Step 1: Identify correct order of modifiers

   The $each modifier must come before $slice inside $push.

2. Step 2: Validate syntax correctness

   { $push: { scores: { $each: [90, 85], $slice: 3 } } } correctly uses $each: [90, 85] followed by $slice: 3 to limit array size.

3. Final Answer:

   { $push: { scores: { $each: [90, 85], $slice: 3 } } } -> Option A

4. Quick Check:

   $each before $slice [OK]

Hint: Put $each before $slice inside $push [OK]

Common Mistakes:

• Swapping order of $each and $slice
• Using arrays instead of numbers for $slice
• Placing $slice outside $push

3. Given the document { _id: 1, items: [1, 2, 3, 4] }, what will be the items array after running this update?
db.collection.updateOne({ _id: 1 }, { $push: { items: { $each: [5, 6], $slice: -4 } } })

medium

A. [3, 4, 5, 6]

B. [1, 2, 3, 4, 5, 6]

C. [2, 3, 4, 5]

D. [5, 6]

Solution

1. Step 1: Understand $each and $slice with negative value

   $each adds [5, 6] to the array. $slice: -4 keeps the last 4 elements of the array.

2. Step 2: Calculate new array content

   Original array: [1, 2, 3, 4]
   After push: [1, 2, 3, 4, 5, 6]
   Apply $slice: -4: keep last 4 elements -> [3, 4, 5, 6]

3. Final Answer:

   [3, 4, 5, 6] -> Option A

4. Quick Check:

   Negative $slice keeps last elements [OK]

Hint: Negative $slice keeps last N elements [OK]

Common Mistakes:

• Ignoring $slice and keeping all elements
• Using positive $slice logic incorrectly
• Removing new elements instead of old

4. You run this update:
db.collection.updateOne({ _id: 2 }, { $push: { logs: { $each: ["a", "b"], $slice: 1 } } })
But the array logs ends up with 1 element that is an old one instead of a newly pushed one. What is the likely mistake?

medium

A. Not specifying $position causes elements to append incorrectly

B. Missing $each causes syntax error

C. Using $slice with a negative number causes array to clear

D. Using positive $slice keeps first elements instead of newest ones

Solution

1. Step 1: Understand positive $slice behavior

   Positive $slice: 1 keeps only the first element of the array after push.

2. Step 2: Why old elements remain

   Positive $slice: 1 keeps the first (oldest) element of the resulting array since new elements are appended to the end.

3. Step 3: Common confusion

   Positive $slice keeps elements from the start, so if you want to keep the latest elements, use negative $slice.

4. Final Answer:

   Using positive $slice keeps first elements instead of newest ones -> Option D

5. Quick Check:

   Positive $slice keeps start elements [OK]

Hint: Use negative $slice to keep newest elements [OK]

Common Mistakes:

• Confusing positive and negative $slice behavior
• Omitting $each inside $push
• Expecting $slice to remove elements before push

5. You want to keep only the 5 most recent messages in a chat array after adding new messages ["msg4", "msg5"]. The current array is ["msg1", "msg2", "msg3", "msg4"]. Which update query correctly achieves this?

hard

A. db.collection.updateOne({ _id: 1 }, { $push: { chat: { $each: ["msg4", "msg5"], $slice: 5 } } })

B. db.collection.updateOne({ _id: 1 }, { $push: { chat: { $each: ["msg4", "msg5"], $slice: -5 } } })

C. db.collection.updateOne({ _id: 1 }, { $push: { chat: { $each: ["msg4", "msg5"], $slice: 2 } } })

D. db.collection.updateOne({ _id: 1 }, { $push: { chat: { $each: ["msg4", "msg5"], $slice: -2 } } })

Solution

1. Step 1: Understand the goal

   We want to add two new messages and keep only the 5 most recent messages in the array.

2. Step 2: Use $each and negative $slice

   $each adds multiple items. Negative $slice: -5 keeps the last 5 elements, which are the most recent.

3. Step 3: Check options

   db.collection.updateOne({ _id: 1 }, { $push: { chat: { $each: ["msg4", "msg5"], $slice: -5 } } }) uses $slice: -5 correctly. db.collection.updateOne({ _id: 1 }, { $push: { chat: { $each: ["msg4", "msg5"], $slice: 5 } } }) keeps first 5 elements (oldest), C keeps only 2 elements, D keeps last 2 elements, which is too few.

4. Final Answer:

   db.collection.updateOne({ _id: 1 }, { $push: { chat: { $each: ["msg4", "msg5"], $slice: -5 } } }) -> Option B

5. Quick Check:

   Negative $slice keeps newest 5 elements [OK]

Hint: Use negative $slice with $each to keep recent items [OK]

Common Mistakes:

• Using positive $slice which keeps oldest items
• Setting $slice too small and losing messages
• Forgetting $each when adding multiple items