Arrays & Nested Documents

$elemMatch

When array conditions must hold on the same element — the distinction interviews keep asking about.

IntermediateAbout 7 minutes

$elemMatch says: one element of the array must satisfy all of these conditions. Same order as the last lesson. Same trap. This is the fix.

{
  userId: ObjectId("64a1b2c3d4e5f6a7b8c90001"),
  amount: 4900,
  status: "completed",
  items: [
    { name: "Mechanical Keyboard", quantity: 1, unitPrice: 4500 },
    { name: "USB-C Cable", quantity: 2, unitPrice: 200 }
  ]
}

The order from before — keyboard ₹4500 qty 1, cable ₹200 qty 2

Any elements (dotted)

items.unitPrice > 500 (keyboard)
items.quantity = 2 (cable)
Order matches — probably wrong

Same element ($elemMatch)

One line: unitPrice > 500 AND quantity 2
This order does not match
A 2× keyboard at ₹4500 would

db.orders.find({
  items: {
    $elemMatch: {
      unitPrice: { $gt: 500 },
      quantity: 2
    }
  }
})

The query you meant

You do not need $elemMatch for a single condition: { "items.name": "USB-C Cable" } is enough. You need it when two or more conditions must share an element. You also need it for a single complex operator on one element that would otherwise be ambiguous — { items: { $elemMatch: { quantity: { $gte: 2 } } } } is valid, though { "items.quantity": { $gte: 2 } } already means 'some element'.

db.orders.find(
  { items: { $elemMatch: { name: "USB-C Cable" } } },
  { _id: 0, amount: 1, items: { $elemMatch: { name: "USB-C Cable" } } }
)

Projection: return only the matching item(s), not the whole array

Interview question

How do you query a nested field, and how do you match inside an array of objects?

Think about it first.

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