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.inventory.findOneAndUpdate({ sku: 'A100', on_hand: { $gt: 0 } }, { $inc: { on_hand: -1 } }, { returnDocument: 'after' })

  Why this is correct

  findOneAndUpdate atomically matches the filter, applies the decrement, and returns the document. With returnDocument: 'after' the client receives the post-decrement state, and the on_hand: { $gt: 0 } filter prevents negative stock. This single atomic operation satisfies both the conditional decrement and the requirement to return the updated document.

• ✗

  db.inventory.bulkWrite([{ updateOne: { filter: { sku: 'A100', on_hand: { $gt: 0 } }, update: { $inc: { on_hand: -1 } } } }])

  Why it's wrong here

  A bulkWrite performs the conditional atomic decrement correctly, but it returns only a summary of matched and modified counts, not the resulting document. The requirement to return the post-update document to the client is unmet. Also, a bulk write of a single operation offers no concurrency advantage over findOneAndUpdate here.

• ✗

  db.inventory.find({ sku: 'A100', on_hand: { $gt: 0 } }) followed by db.inventory.updateOne({ sku: 'A100' }, { $inc: { on_hand: -1 } })

  Why it's wrong here

  Splitting the read and the write creates a race condition. Between the find and the update, another session can decrement on_hand to zero, so the update still runs and drives stock negative. The two operations are not atomic together, and the update does not return the resulting document either, failing both requirements.

• ✗

  db.inventory.updateOne({ sku: 'A100', on_hand: { $gt: 0 } }, { $inc: { on_hand: -1 } }, { returnDocument: 'after' })

  Why it's wrong here

  updateOne returns a result object with matchedCount and modifiedCount but does not return the post-update document. The returnDocument option belongs to findOneAndUpdate, not updateOne, so it is ignored. The decrement itself is atomic, but the requirement to receive the resulting document is not met by this call.