What does the following $slice query return using the following command? db.posts.find( {}, { comments: { $slice: [ -10, 5 ] } } )

Question

Options

• AReturns 5 comments, beginning with the last 10 items
• BReturns 10 comments, beginning with the last
• CReturns 10 comments, beginning with the first
• DReturns 5 comments, beginning with the first 10 items

• A
  83% (25)
• B
  3% (1)
• C
  10% (3)
• D
  3% (1)

Explanation

Option A is correct because $slice: [-10, 5] uses a two-element array where the first value is the skip offset and the second is the count to return. A negative first value counts from the end of the array, so -10 positions the cursor at the 10th-from-last element, and 5 returns the next 5 elements from that point.

Why the distractors are wrong:

• B confuses the two parameters - -10 is not a count of documents to return, it's a starting offset from the end.
• C is doubly wrong: it misreads both the negative sign (which means "from the end," not "from the beginning") and the count.
• D flips the semantics - 5 is not a skip amount and -10 is not a count; the roles of the two numbers are reversed.

Memory tip: Think of $slice: [skip, limit] like a window on an array. A negative skip slides the window from the tail of the array; a positive limit defines how wide the window is. So [-10, 5] = "start 10 from the end, grab 5."

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