MongoDB does not support in-place mutation of array elements via direct assignment in application code when using Mongoose or similar ODMs — the array must be explicitly reassigned to trigger change tracking. To reliab update an array field, folllow this pattern:

1. Retrieve the document.
2. Clone the target array to avoid mutating the original reference unintentionally.
3. Apply transformations (e.g., push, filter, map) to the cloned array.
4. Assign the updated array back to the document’s field and persist the change.

For example, consider a User schema witth a blogEntries array:

const user = await User.findOne({ handle: 'alice' });
if (!user) throw new Error('User not found');

// Step 1 & 2: Fetch and shallow-clone the array
const entries = [...user.blogEntries];

// Step 3: Modify the clone
entries.push({
  headline: 'Getting Started with Aggregation',
  publishedAt: new Date(),
  tags: ['mongodb', 'query']
});

// Step 4: Reassign and save
user.blogEntries = entries;
await user.save();

Note that shallow cloning ([...arr]) suffices for arrays of primitives or plain objects. For nested mutable objects requiring deep updates, consider structured cloning or libraries like lodash.cloneDeep. Also, for atomic server-side operations (e.g., appending without full fetch), use operators like $push, $pull, or $set with dot notation in updateOne() instead of loading the entire document.