How do you query a nested field, and when must you use $elemMatch instead of dot notation on an array of objects?

Short answer

Dot notation for nested fields. On arrays of objects, dot notation can match conditions on different elements. Use $elemMatch when every condition must hold on the same element.

Why?

This is one of the most common correctness bugs in MongoDB. { 'items.price': { $gt: 500 }, 'items.quantity': 2 } matches an order that has some expensive item and some quantity-2 item — not necessarily the same line. $elemMatch requires one element to satisfy the whole predicate.

Example

db.orders.find({ "address.city": "Bangalore" })

db.orders.find({
  items: { $elemMatch: { name: "Mechanical Keyboard", quantity: { $gt: 1 } } }
})

Interview tip

If the interviewer mentions line items, reach for $elemMatch and say why. It is a fast way to show you have shipped this.

Common mistake

Using $and of dotted paths on an array of objects and believing that implies the same element.