Updating Documents — updateOne, updateMany and Update Operators

Updating documents is one of the most nuanced operations in MongoDB. Unlike SQL’s UPDATE SET, MongoDB has a rich set of update operators that let you modify specific fields, increment counters, push items onto arrays, and pull items out — all atomically and without fetching the document first. Getting updates wrong leads to data loss (accidentally replacing documents), stale data (not running validators), or race conditions (non-atomic read-modify-write). This lesson covers every update pattern you will use in the MERN Blog, from simple field changes to atomic array modifications.

Update Methods Overview #

The $set Operator — Update Specific Fields #

// ── updateOne with $set ────────────────────────────────────────────────────────
await Post.updateOne(
  { _id: postId },                      // filter — which document to update
  { $set: { published: true,            // update — what to change (only these fields)
             publishedAt: new Date() } }
);

// ── findByIdAndUpdate with $set — returns the updated document ─────────────────
const updated = await Post.findByIdAndUpdate(
  postId,
  { $set: { title: 'Updated Title', body: 'Updated content' } },
  { new: true, runValidators: true }    // new: true = return updated doc
);

// ── Update only the fields provided in req.body ────────────────────────────────
// Build a $set object from only the fields that were sent
const { title, body, excerpt, tags, coverImage } = req.body;
const updates = {};
if (title      !== undefined) updates.title      = title;
if (body       !== undefined) updates.body       = body;
if (excerpt    !== undefined) updates.excerpt    = excerpt;
if (tags       !== undefined) updates.tags       = tags;
if (coverImage !== undefined) updates.coverImage = coverImage;

const post = await Post.findByIdAndUpdate(
  postId,
  { $set: updates },
  { new: true, runValidators: true }
);

Common Update Operators #

// ── $set — set specific fields ─────────────────────────────────────────────────
Post.updateOne({ _id: id }, { $set: { published: true } });

// ── $unset — remove a field entirely ──────────────────────────────────────────
Post.updateOne({ _id: id }, { $unset: { excerpt: "" } });
// The value in $unset does not matter — the field is removed regardless

// ── $inc — increment or decrement a number ────────────────────────────────────
Post.updateOne({ _id: id }, { $inc: { viewCount: 1 } });   // +1
Post.updateOne({ _id: id }, { $inc: { viewCount: -1 } });  // -1

// ── $push — add an element to an array ────────────────────────────────────────
Post.updateOne({ _id: id }, { $push: { tags: 'mongodb' } });

// ── $addToSet — add to array only if not already present (no duplicates) ──────
Post.updateOne({ _id: id }, { $addToSet: { likedBy: userId } });
// If userId is already in likedBy — no change. Otherwise appended.

// ── $pull — remove matching elements from an array ────────────────────────────
Post.updateOne({ _id: id }, { $pull: { tags: 'old-tag' } });
Post.updateOne({ _id: id }, { $pull: { likedBy: userId } }); // unlike a post

// ── $rename — rename a field ──────────────────────────────────────────────────
Post.updateMany({}, { $rename: { 'content': 'body' } }); // rename all at once

// ── Combining multiple operators ──────────────────────────────────────────────
Post.updateOne({ _id: id }, {
  $set:  { published: true, publishedAt: new Date() },
  $inc:  { viewCount: 1 },
  $push: { tags: 'featured' },
});

Upsert — Update or Create #

// upsert: true — update if found, insert if not found
// Useful for "set if exists, create if not" patterns

await UserSettings.findOneAndUpdate(
  { user: userId },                         // filter
  { $set: { theme: 'dark', language: 'en' } }, // update
  { upsert: true, new: true }               // create if missing
);
// If settings document exists → updates theme and language
// If not → creates { user: userId, theme: 'dark', language: 'en' }

Fetch-Modify-Save Pattern #

// When you need conditional update logic or to run all hooks:
const post = await Post.findById(postId);
if (!post) throw new AppError('Post not found', 404);
if (post.author.toString() !== req.user.id) throw new AppError('Not authorised', 403);

// Modify the document in JavaScript
post.title     = req.body.title   || post.title;
post.body      = req.body.body    || post.body;
post.published = req.body.published !== undefined ? req.body.published : post.published;

if (!post.published && req.body.published === true) {
  post.publishedAt = new Date(); // set publishedAt only on first publish
}

// Saves the modified document — runs all validators and pre/post save hooks
const saved = await post.save();
res.json({ success: true, data: saved });

Common Mistakes #

Mistake 1 — Replacement update instead of $set #

❌ Wrong — passing a plain object replaces the entire document:

await Post.updateOne({ _id: id }, { title: 'New Title' });
// The document is now ONLY: { _id: id, title: 'New Title' }
// All other fields (body, author, tags, published...) are GONE

✅ Correct — always use an update operator:

await Post.updateOne({ _id: id }, { $set: { title: 'New Title' } }); // ✓

Mistake 2 — Not passing { new: true } to findByIdAndUpdate #

❌ Wrong — the returned document is the state BEFORE the update:

const post = await Post.findByIdAndUpdate(id, { $set: { published: true } });
console.log(post.published); // false — pre-update state returned by default

✅ Correct — pass { new: true } to get the updated document:

const post = await Post.findByIdAndUpdate(
  id, { $set: { published: true } }, { new: true }
);
console.log(post.published); // true ✓

Mistake 3 — Not using $addToSet for arrays that should have unique values #

❌ Wrong — using $push allows duplicates in an array that should be a set:

// User likes the same post twice
await Post.updateOne({ _id: id }, { $push: { likedBy: userId } });
await Post.updateOne({ _id: id }, { $push: { likedBy: userId } });
// likedBy: [userId, userId] — duplicate! Like count is wrong

✅ Correct — use $addToSet for unique membership:

await Post.updateOne({ _id: id }, { $addToSet: { likedBy: userId } }); // idempotent ✓

Quick Reference #