CCNA CRUD Operations Questions

findOneAndUpdate performs the conditional match, the $inc decrement, and the return of the document in one atomic operation. The on_hand: { $gt: 0 } filter guards against negative inventory under concurrency, and returnDocument: 'after' supplies the resulting state. Separating read and write, or using updateOne or bulkWrite, either introduces races or fails to return the document.

Cursors are lazy and fetch documents in batches during iteration, and they close automatically when exhausted but require explicit close() if iteration stops early. They do not eagerly load all matches, do not automatically re-execute on concurrent writes, and do not guarantee any result order without an explicit sort. These properties affect both memory use and server resource management.

The $pull operator deletes array elements that match a specified condition or value, which is exactly what removing a single embedded subdocument by its trackingCode requires. $pop and $pullAll operate positionally or on exact element values and cannot target a nested field match, while $unset would leave a null placeholder instead of deleting the element.

$inc performs atomic relative addition, so a negative value decrements the field and creates it when absent. $set replaces the value outright, $mul scales instead of subtracting, and $push targets arrays rather than numeric fields, so none of those meet the decrement requirement.

The $all operator selects the documents where the value of a field is an array that contains all the specified elements. This is essential when searching arrays for multiple matching criteria without enforcing strict positional indexing or exact array equality.

The requirement is a filtered, multi-document field update, which maps directly to updateMany with the $set operator. The filter expresses both conditions, and $set preserves the existing document structure while adding or overwriting deliveredAt. The single-document helpers leave other matches untouched, and replacement operations would remove the untouched fields.

Write concern w:1 confirms the write only on the primary. If that primary fails before the operation reaches any secondary, the new primary never has the change, and the old primary rolls it back upon rejoining, so an acknowledged increment can disappear. Stronger write concern such as majority prevents this by requiring replication before acknowledgement.

The optimal index follows the equality-sort-range rule: equality fields first, then the sort field. Because the query filters on deviceId with equality and sorts on recordedAt descending, a compound index { deviceId: 1, recordedAt: -1 } lets MongoDB use the prefix for filtering and read entries in the requested sort order, avoiding the blocking in-memory sort and its memory overhead.

An ordered insertMany() writes documents in sequence and stops at the first error, matching the requirement. Unordered insertion continues past failures, insertOne() handles only one document, and bulkWrite() is intended for mixed operations and here omits a document.

insertOne is the purpose-built method for adding a single document and returns an InsertOneResult whose insertedId field exposes the generated or supplied _id. This satisfies the requirement to obtain the identifier immediately, without a follow-up find or relying on deprecated methods such as save or update.

Projection operates in either inclusion or exclusion mode, never both, with _id as the sole exception that can be suppressed alongside included fields. Projection only shapes query results and does not alter stored documents, and _id can indeed be excluded.

The $pull operator is designed to remove array elements that match a specified condition, so using it with updateMany and a carrier filter deletes only the cancelled tracking_history entries from every DHL shipment. Operations like deleteMany or $pop either remove whole documents or the wrong array elements, and $unset cannot filter by element values.

Multi-document transactions are the correct tool when atomicity must span more than one document. Starting a transaction on a session, performing the debit and credit updates with that session, and committing ensures the operations either all commit or all abort. On a replica set this provides the all-or-nothing behavior the banking scenario demands.

Deleting a subset of documents based on field equality requires deleteMany() with the filter, which removes all matches and reports deletedCount. deleteOne() would remove only one record, the deprecated remove() is unavailable on current servers, and drop() eliminates the entire collection rather than matching documents.

The filtered positional operator with arrayFilters matches subdocuments by condition, so only the email element changes. A hardcoded index may hit the wrong entry, the plain positional operator needs an array condition in the query, and replacing the whole array deletes other contacts.

The insert_one method inserts a single document and returns an InsertOneResult whose inserted_id exposes the generated identifier. It does not upsert, does not defer writes to connection close, and does not require the application to provide _id. Understanding this return object matters because applications commonly need the generated _id to link related documents or respond to clients.

The upsert option instructs MongoDB to insert a new document if no existing document matches the query filter. When combined with update operators, the final document is constructed by merging the query filter with the update operators, which is crucial for idempotent bootstrapping patterns.

The $pull operator is designed to remove array elements that match a specified condition, making it ideal for removing subdocuments based on a field value. It works with updateMany to affect all documents. Other operators like $unset, $pop, and $pullAll do not support conditional removal of subdocuments based on a field, so they would not achieve the desired result.

Projection is the second parameter of find, and mixing inclusion values with _id: 0 is the standard way to return a subset of fields without the identifier. Passing fields as the first argument turns them into a filter, and using exclusion values in a projection drops the very fields the developer needs.

The correct operation must update multiple open tickets that lack a priority field, add the field without overwriting existing values, and avoid creating new documents. Using updateMany with a filter that includes $exists: false and a $set modifier satisfies all conditions. Operators like $setOnInsert or upsert options are designed for insert scenarios and would either do nothing or risk unintended inserts, while replaceOne would remove other fields.

An equality filter on a single boolean field is best served by a single-field index on that field, which allows the planner to seek directly to the matching entries and fetch only relevant documents. A compound index led by _id, a text index, or a sparse variant cannot provide that access path, because the query does not supply a usable prefix or field type for those structures.

Query plan selection is cost-based. With a few hundred documents, the estimated cost of a full collection scan can be lower than the cost of using a multikey index plus document fetches, so the planner may reasonably choose COLLSCAN. As the collection grows, or with a more selective predicate, the same index is likely to be selected.

The $push operator is specifically designed to append values to an array. It adds the new comment subdocument to the 'comments' array without affecting existing elements. The other options either prevent duplicates, overwrite the array, or use an invalid operator. $push is the standard and correct choice for this scenario.

deleteMany with a filter removes all matching documents and reports how many were deleted via deletedCount. deleteOne only removes the first match, drop destroys the whole collection, and remove is deprecated. When a selective deletion is needed and the filter is indexed, deleteMany preserves unrelated documents and existing indexes.

The correct queries must combine accountId, amount greater than 1000, and status in a list using logical AND. The implicit AND version and the explicit $and version both achieve this. The other options either use invalid syntax, combine conditions with OR instead of AND, or use the wrong comparison operator, leading to incorrect results.

The correct approach is updateOne with a filter on active, $inc for the numeric change, and $set for the timestamp. Using update operators applies atomic field-level changes to one matched document. Omitting operators triggers replacement semantics, updateMany affects too many documents, and replaceOne forbids modifier keys, so each alternative violates part of the requirement.