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.updateOne({ sku: "A1" }, { $push: { quantity: -5 } })

  Why it's wrong here

  $push appends a value to an array field. Applying it to a numeric quantity would either error because the field is not an array or convert the field's type unexpectedly. It cannot decrement a number, so it fails the requirement entirely and risks corrupting the document's schema.

• ✗

  db.inventory.updateOne({ sku: "A1" }, { $mul: { quantity: -5 } })

  Why it's wrong here

  $mul multiplies the field by the given value, so -5 would multiply quantity by negative five, producing a wildly incorrect result and potentially a negative stock figure. It performs scaling, not subtraction. This operator is meant for proportional changes such as applying a percentage factor, not for fixed decrements.

• ✓

  db.inventory.updateOne({ sku: "A1" }, { $inc: { quantity: -5 } })

  Why this is correct

  $inc adds the given value to the field, so -5 decrements quantity by five. If the field does not exist, $inc creates it and sets it to the specified negative value, satisfying the requirement. updateOne() targets the single matching item, which is correct for adjusting one SKU's stock.

• ✗

  db.inventory.updateOne({ sku: "A1" }, { $set: { quantity: -5 } })

  Why it's wrong here

  $set assigns an absolute value, so this would overwrite quantity with -5 regardless of its current amount rather than subtracting five. It also does not compute a relative change, which is the core requirement. This is a common mistake when the intent is arithmetic adjustment rather than replacement.