What does the following query do when performed on the posts collection? db.posts.update({_id:l},{Title:This is post with ID 1"})

Question

Options

• ASyntax error
• BReplaces the complete document with _id as 1 with the document specified in second parameter
• CUpdates the Title of the post
• DUpdating a document is possible only with $set

• A
  3% (1)
• B
  85% (28)
• C
  9% (3)
• D
  3% (1)

Explanation

In MongoDB's update() method, when the second parameter contains no update operators (like $set), MongoDB treats it as a replacement document - it discards all existing fields and replaces the entire matched document with the new one (preserving _id). That makes B correct: the document with _id: 1 is fully replaced, not partially updated.

Why the distractors are wrong:

• A (Syntax error) - The query is syntactically valid MongoDB; it will execute without error.
• C (Updates the Title) - This is the most tempting wrong answer. Updating only a field requires $set: {$set: {Title: "..."}}. Without $set, the entire document is replaced, so any other fields (e.g., Body, Author) are lost.
• D (Only $set can update) - False; MongoDB supports many update operators ($inc, $unset, $push, etc.), and replacement-style updates (no operator) are also valid.

Memory tip: Think of the second parameter as two modes - operator mode (starts with $) modifies fields in place, while replacement mode (no $) swaps the whole document. If you don't see a $, the document gets replaced, not patched.

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