C100DBA 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.products.updateOne( { _id: 1, "inventory.sku": "B2" }, { $inc: { "inventory.0.qty": 3 } } )

  Why it's wrong here

  The path `inventory.0.qty` always targets the first element of the array, regardless of which subdocument matched the query. If "B2" is not at index 0, the wrong subdocument is updated. The positional operator is required to update the element that actually matched the query condition.

• ✗

  db.products.updateOne( { _id: 1, "inventory.sku": "B2" }, { $set: { "inventory.$.qty": "3" } } )

  Why it's wrong here

  Using `$set` with the string "3" changes the field type from a number to a string, which breaks numeric operations and query comparisons. The requirement is to increase the existing numeric quantity by 3, so `$inc` with a numeric value is appropriate; `$set` with a string does not perform arithmetic.

• ✓

  db.products.updateOne( { _id: 1, "inventory.sku": "B2" }, { $inc: { "inventory.$.qty": 3 } } )

  Why this is correct

  This correctly uses the positional `$` operator. The query identifies the array element whose `sku` equals "B2", and the positional operator updates the `qty` field of the matched array element only. This is the standard MongoDB pattern for updating a specific embedded subdocument in an array without replacing the whole array.

• ✗

  db.products.updateOne( { _id: 1 }, { $inc: { "inventory.qty": 3 } } )

  Why it's wrong here

  Without the positional operator or an array filter, the path `inventory.qty` targets the `qty` field of the array itself, not of its elements. MongoDB will not match a scalar `qty` field on the array, so the `$inc` has no effect on the embedded subdocuments, leaving `qty` values unchanged.