C100DEV CRUD Operations Practice Question

Answer analysis

Option-by-option breakdown

For each option: why learners choose it and why it is or isn't the right answer here.

• ✗

  db.posts.updateOne({ _id: postId }, { $set: { comments: { author: 'user1', text: 'Great post!' } } })

  Why it's wrong here

  $set replaces the entire 'comments' field with the new subdocument, turning the array into a single object. This would destroy all existing comments. It does not append; it overwrites. This is incorrect and would lead to data loss.

• ✓

  db.posts.updateOne({ _id: postId }, { $push: { comments: { author: 'user1', text: 'Great post!' } } })

  Why this is correct

  The $push operator appends a specified value to an array. Here, it adds the new comment subdocument to the 'comments' array. This is the correct and efficient way to add an element to an array without needing to read and rewrite the entire document. It works atomically.

• ✗

  db.posts.updateOne({ _id: postId }, { $addToSet: { comments: { author: 'user1', text: 'Great post!' } } })

  Why it's wrong here

  $addToSet adds an element to an array only if it does not already exist. If the same comment (exact match) is already present, it will not add it. The requirement is to add a new comment regardless of duplicates, as comments can be repeated. $addToSet would prevent duplicate comments with identical content, which is not desired.

• ✗

  db.posts.updateOne({ _id: postId }, { $concat: { comments: { author: 'user1', text: 'Great post!' } } })

  Why it's wrong here

  $concat is not a valid update operator in MongoDB. It is an aggregation operator used for string concatenation in the aggregation pipeline. Using it in an update would cause an error. The correct operator for appending to an array is $push.