$slice modifier with $push in MongoDB - Step-by-Step Execution

Concept Flow - $slice modifier with $push

Start with existing array

↓

Apply $push to add element

↓

Apply $slice to trim array

↓

Update document with new array

↓

End with modified array

This flow shows how MongoDB adds an element to an array and then trims it using $slice to keep only a certain number of elements.

Execution Sample

MongoDB

db.collection.updateOne(
  { _id: 1 },
  { $push: { scores: { $each: [85], $slice: -3 } } }
)

Adds 85 to the 'scores' array and keeps only the last 3 elements.

Execution Table

Step	Existing Array	Operation	Array After Operation	Explanation
1	[70, 80]	Start	[70, 80]	Initial array in document
2	[70, 80]	$push 85	[70, 80, 85]	Added 85 to the end
3	[70, 80, 85]	$slice -3	[70, 80, 85]	Array length is 3, no trimming needed
4	[70, 80, 85]	Update document	[70, 80, 85]	Document updated with new array
5	[70, 80, 85]	Next $push 90	[70, 80, 85, 90]	Added 90 to the end
6	[70, 80, 85, 90]	$slice -3	[80, 85, 90]	Trimmed to last 3 elements
7	[80, 85, 90]	Update document	[80, 85, 90]	Document updated with trimmed array
8	[80, 85, 90]	End	[80, 85, 90]	Final array after operations

💡 No more operations; array trimmed to last 3 elements after each push

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 5	After Step 6	Final
scores	[70, 80]	[70, 80, 85]	[70, 80, 85]	[70, 80, 85, 90]	[80, 85, 90]	[80, 85, 90]

Key Moments - 2 Insights

Why does the array get trimmed after adding a new element?

Because the $slice modifier limits the array size after $push, keeping only the specified number of elements (last 3 here), as shown in steps 3 and 6.

What happens if the array is shorter than the $slice limit before pushing?

The array stays the same length after $push if it doesn't exceed the $slice limit, as in step 3 where the array was 3 elements and $slice -3 kept all.

Visual Quiz - 3 Questions

Look at the execution table, what is the array after step 2?

A [70, 80, 85]

B [70, 80]

C [80, 85]

D [85]

💡 Hint

Check the 'Array After Operation' column for step 2 in the execution_table.

At which step does the array first get trimmed by $slice?

A Step 3

B Step 4

C Step 6

D Step 7

💡 Hint

Look for the first step where the array length decreases after $slice in the execution_table.

If we change $slice to -2, what would be the array after step 6?

A [80, 85, 90]

B [85, 90]

C [70, 80]

D [90]

💡 Hint

Refer to variable_tracker and understand $slice -2 keeps last 2 elements.

Concept Snapshot

$push with $slice adds elements to an array and trims it.
Syntax: {$push: {field: {$each: [values], $slice: n}}}
$slice: positive n keeps first n elements, negative n keeps last n.
Useful to limit array size after adding new items.
Example: $slice: -3 keeps last 3 elements after push.

Full Transcript

This visual execution shows how MongoDB's $push operator combined with the $slice modifier works. We start with an array [70, 80]. When we push 85, the array becomes [70, 80, 85]. Applying $slice: -3 keeps the last 3 elements, so the array stays the same. Next, pushing 90 makes the array [70, 80, 85, 90]. Applying $slice: -3 trims it to [80, 85, 90]. The document updates reflect these changes. This process helps keep arrays at a fixed size by trimming after adding new elements.