$slice modifier with $push in MongoDB - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the $push operator do in MongoDB?

The $push operator adds a new value to an array field in a document. If the field doesn't exist, it creates the array with the new value.

beginner

What is the purpose of the $slice modifier when used with $push?

The $slice modifier limits the size of the array after pushing a new element. It keeps only a specified number of elements from the start or end of the array.

intermediate

How do you keep only the last 3 elements of an array when pushing a new element using $push and $slice?

Use $push with $each to add the element, and $slice: -3 to keep the last 3 elements. Example: { $push: { arr: { $each: [newValue], $slice: -3 } } }

intermediate

What happens if you use $slice: 0 with $push?

Using $slice: 0 removes all elements from the array after pushing, resulting in an empty array.

beginner

Can $slice be used alone without $push in an update operation?

No, $slice is a modifier used only with $push to control the array size after pushing elements. It cannot be used alone.

What does $push do in MongoDB?

A Removes a value from an array

B Adds a value to an array field

C Replaces the entire document

D Creates a new collection

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

How does $slice: -3 affect an array when used with $push?

A Removes all elements

B Keeps the first 3 elements

C Keeps the last 3 elements

D Adds 3 new elements

$slice: -3 keeps only the last 3 elements of the array after pushing.

Which of these is the correct syntax to push a value and limit array size to 5?

A { $slice: 5, $push: { arr: value } }

B { $push: { arr: value, $slice: 5 } }

C { $push: { arr: [value], $limit: 5 } }

D { $push: { arr: { $each: [value], $slice: 5 } } }

Use $each with $push and $slice to limit array size.

What happens if you use $slice: 0 with $push?

A Array becomes empty

B Array keeps all elements

C Array size doubles

D Operation fails

$slice: 0 removes all elements, resulting in an empty array.

Can $slice be used without $push?

A No, it only works with <code>$push</code>

B Yes, but only in aggregation

C Yes, but only with <code>$pull</code>

D Yes, it works alone

$slice is a modifier only for $push operations.

Explain how to use $push with $slice to keep an array's size limited after adding new elements.

Think about adding elements and then trimming the array.

Describe what happens when you use $slice: 0 with $push in a MongoDB update.

Consider what zero means for array length.

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