Updating and Deleting Documents

MongoDB Fundamentals — Updating and Deleting Documents

MongoDB Fundamentals

Chapter 4 · Updating and Deleting Documents

✏️ Updating and Deleting Documents

Chapter 3 covered Create and Read. This chapter finishes off CRUD with Update and Delete — and one behavior worth knowing about carefully before you hit it by accident.

updateOne and updateMany

Both take a filter (which document(s) to change) and an update (what to change). updateOne affects only the first match; updateMany affects every matching document.

Update Operators: $set, $inc, $push

Operator	Meaning
`$set`	Set a field to a specific value (the most common update operator)
`$inc`	Increment (or decrement, with a negative number) a numeric field
`$push`	Append a value onto an array field

$set — Change a Field

db.tasks.updateOne(
  { title: "Write chapter draft" },
  { $set: { done: true } }
)
            

$inc — Adjust a Counter

db.products.updateOne(
  { name: "Notebook" },
  { $inc: { stock: -1 } }
)
// decrements stock by 1 — one sold
            

$push — Append to an Array

db.posts.updateOne(
  { title: "Why I Switched to MongoDB" },
  { $push: { comments: { author: "Dana", text: "Nice writeup!" } } }
)
// adds one new comment to the embedded comments array from Chapter 1's challenge
            

Upserts: Update or Insert

Adding { upsert: true } as a third argument tells MongoDB: if no document matches the filter, insert a new one instead of doing nothing.

An Upsert for a Page-View Counter

db.pageViews.updateOne(
  { page: "/pricing" },
  { $inc: { views: 1 } },
  { upsert: true }
)
// first time this page is viewed: creates { page: "/pricing", views: 1 }
// every time after: increments the existing views field
            

Deleting Documents: deleteOne and deleteMany

Deleting

db.tasks.deleteOne({ title: "Write chapter draft" })
// removes the first matching document

db.tasks.deleteMany({ done: true })
// removes every completed task
            

💻 Coding Challenges

Challenge 1: Update a Single Field

Write an updateOne call that sets a product named "Laptop"'s price field to 799.

Goal: Practice basic $set syntax.

→ Solution

Challenge 2: Append to an Array

Write an updateOne call adding a new tag "sale" to a product's tags array field, for the product named "Notebook".

Goal: Practice $push on an array field.

→ Solution

Challenge 3: Write an Upsert

Write an updateOne call that increments a visitor counter for a user identified by userId: "u42", creating the counter document with { userId: "u42", visits: 1 } if it doesn't exist yet.

Goal: Practice combining $inc with upsert: true.

→ Solution

⚠️ Gotcha: Forgetting $set Silently Replaces the Whole Document

If the second argument to updateOne has no operator like $set at all — just plain fields, e.g. db.users.updateOne({ name: "Alice" }, { email: "new@example.com" }) — MongoDB doesn't merge that field into the existing document. It treats the whole thing as a replacement document, wiping out every other field the original document had. This is exactly the REST PUT-vs-PATCH distinction from the API Types & Design course's HTTP chapter, applied to MongoDB: a bare update document behaves like a full PUT replacement, while wrapping it in $set behaves like a PATCH partial update. Always use $set (or another update operator) unless you genuinely intend to replace the entire document.

🎯 What's Next

The next chapter goes deeper into design itself: The Document Data Model & Schema Design — embedding vs. referencing, denormalization philosophy, and modeling one-to-many and many-to-many relationships as documents.